diff --git a/Lib/aifc.py b/Lib/aifc.py
index d8b7fab..9ac710f 100644
--- a/Lib/aifc.py
+++ b/Lib/aifc.py
@@ -778,7 +778,7 @@
 
     def _ensure_header_written(self, datasize):
         if not self._nframeswritten:
-            if self._comptype in ('ULAW', 'ALAW'):
+            if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw'):
                 if not self._sampwidth:
                     self._sampwidth = 2
                 if self._sampwidth != 2:
@@ -844,7 +844,7 @@
         if self._datalength & 1:
             self._datalength = self._datalength + 1
         if self._aifc:
-            if self._comptype in ('ULAW', 'ALAW'):
+            if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw'):
                 self._datalength = self._datalength // 2
                 if self._datalength & 1:
                     self._datalength = self._datalength + 1
@@ -852,7 +852,10 @@
                 self._datalength = (self._datalength + 3) // 4
                 if self._datalength & 1:
                     self._datalength = self._datalength + 1
-        self._form_length_pos = self._file.tell()
+        try:
+            self._form_length_pos = self._file.tell()
+        except (AttributeError, IOError):
+            self._form_length_pos = None
         commlength = self._write_form_length(self._datalength)
         if self._aifc:
             self._file.write('AIFC')
@@ -864,7 +867,8 @@
         self._file.write('COMM')
         _write_ulong(self._file, commlength)
         _write_short(self._file, self._nchannels)
-        self._nframes_pos = self._file.tell()
+        if self._form_length_pos is not None:
+            self._nframes_pos = self._file.tell()
         _write_ulong(self._file, self._nframes)
         if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw', 'G722'):
             _write_short(self._file, 8)
@@ -875,7 +879,8 @@
             self._file.write(self._comptype)
             _write_string(self._file, self._compname)
         self._file.write('SSND')
-        self._ssnd_length_pos = self._file.tell()
+        if self._form_length_pos is not None:
+            self._ssnd_length_pos = self._file.tell()
         _write_ulong(self._file, self._datalength + 8)
         _write_ulong(self._file, 0)
         _write_ulong(self._file, 0)
diff --git a/Lib/test/audiotests.py b/Lib/test/audiotests.py
index e378277..93576e6 100644
--- a/Lib/test/audiotests.py
+++ b/Lib/test/audiotests.py
@@ -6,6 +6,13 @@
 import sys
 import base64
 
+class UnseekableIO(io.FileIO):
+    def tell(self):
+        raise io.UnsupportedOperation
+
+    def seek(self, *args, **kwargs):
+        raise io.UnsupportedOperation
+
 def fromhex(s):
     return base64.b16decode(s.replace(' ', ''))
 
@@ -133,6 +140,62 @@
             self.assertEqual(testfile.read(13), b'ababagalamaga')
             self.check_file(testfile, self.nframes, self.frames)
 
+    def test_unseekable_read(self):
+        f = self.create_file(TESTFN)
+        f.setnframes(self.nframes)
+        f.writeframes(self.frames)
+        f.close()
+
+        with UnseekableIO(TESTFN, 'rb') as testfile:
+            self.check_file(testfile, self.nframes, self.frames)
+
+    def test_unseekable_write(self):
+        with UnseekableIO(TESTFN, 'wb') as testfile:
+            f = self.create_file(testfile)
+            f.setnframes(self.nframes)
+            f.writeframes(self.frames)
+            f.close()
+            self.fout = None
+
+        self.check_file(TESTFN, self.nframes, self.frames)
+
+    def test_unseekable_incompleted_write(self):
+        with UnseekableIO(TESTFN, 'wb') as testfile:
+            testfile.write(b'ababagalamaga')
+            f = self.create_file(testfile)
+            f.setnframes(self.nframes + 1)
+            try:
+                f.writeframes(self.frames)
+            except IOError:
+                pass
+            try:
+                f.close()
+            except IOError:
+                pass
+
+        with open(TESTFN, 'rb') as testfile:
+            self.assertEqual(testfile.read(13), b'ababagalamaga')
+            self.check_file(testfile, self.nframes + 1, self.frames)
+
+    def test_unseekable_overflowed_write(self):
+        with UnseekableIO(TESTFN, 'wb') as testfile:
+            testfile.write(b'ababagalamaga')
+            f = self.create_file(testfile)
+            f.setnframes(self.nframes - 1)
+            try:
+                f.writeframes(self.frames)
+            except IOError:
+                pass
+            try:
+                f.close()
+            except IOError:
+                pass
+
+        with open(TESTFN, 'rb') as testfile:
+            self.assertEqual(testfile.read(13), b'ababagalamaga')
+            framesize = self.nchannels * self.sampwidth
+            self.check_file(testfile, self.nframes - 1, self.frames[:-framesize])
+
 
 class AudioTestsWithSourceFile(AudioTests):
 
diff --git a/Lib/test/test_aifc.py b/Lib/test/test_aifc.py
index a312d31..cfc491d 100644
--- a/Lib/test/test_aifc.py
+++ b/Lib/test/test_aifc.py
@@ -9,10 +9,14 @@
 import aifc
 
 
-class AifcPCM8Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
+class AifcTest(audiotests.AudioWriteTests,
+               audiotests.AudioTestsWithSourceFile):
     module = aifc
+    close_fd = True
+    test_unseekable_read = None
+
+
+class AifcPCM8Test(AifcTest, unittest.TestCase):
     sndfilename = 'pluck-pcm8.aiff'
     sndfilenframes = 3307
     nchannels = 2
@@ -27,13 +31,9 @@
       11FA 3EFB BCFC 66FF CF04 4309 C10E 5112 EE17 8216 7F14 8012 \
       490E 520D EF0F CE0F E40C 630A 080A 2B0B 510E 8B11 B60E 440A \
       """)
-    close_fd = True
 
 
-class AifcPCM16Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = aifc
+class AifcPCM16Test(AifcTest, unittest.TestCase):
     sndfilename = 'pluck-pcm16.aiff'
     sndfilenframes = 3307
     nchannels = 2
@@ -50,13 +50,9 @@
       EEE21753 82071665 7FFF1443 8004128F 49A20EAF 52BB0DBA EFB40F60 CE3C0FBF \
       E4B30CEC 63430A5C 08C80A20 2BBB0B08 514A0E43 8BCF1139 B6F60EEB 44120A5E \
       """)
-    close_fd = True
 
 
-class AifcPCM24Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = aifc
+class AifcPCM24Test(AifcTest, unittest.TestCase):
     sndfilename = 'pluck-pcm24.aiff'
     sndfilenframes = 3307
     nchannels = 2
@@ -79,13 +75,9 @@
       E4B49C0CEA2D 6344A80A5A7C 08C8FE0A1FFE 2BB9860B0A0E \
       51486F0E44E1 8BCC64113B05 B6F4EC0EEB36 4413170A5B48 \
       """)
-    close_fd = True
 
 
-class AifcPCM32Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = aifc
+class AifcPCM32Test(AifcTest, unittest.TestCase):
     sndfilename = 'pluck-pcm32.aiff'
     sndfilenframes = 3307
     nchannels = 2
@@ -108,13 +100,9 @@
       E4B49CC00CEA2D90 6344A8800A5A7CA0 08C8FE800A1FFEE0 2BB986C00B0A0E00 \
       51486F800E44E190 8BCC6480113B0580 B6F4EC000EEB3630 441317800A5B48A0 \
       """)
-    close_fd = True
 
 
-class AifcULAWTest(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = aifc
+class AifcULAWTest(AifcTest, unittest.TestCase):
     sndfilename = 'pluck-ulaw.aifc'
     sndfilenframes = 3307
     nchannels = 2
@@ -133,7 +121,6 @@
       """)
     if sys.byteorder != 'big':
         frames = audiotests.byteswap2(frames)
-    close_fd = True
 
 
 class AifcMiscTest(audiotests.AudioTests, unittest.TestCase):
diff --git a/Lib/test/test_sunau.py b/Lib/test/test_sunau.py
index 64cbebd..f682868 100644
--- a/Lib/test/test_sunau.py
+++ b/Lib/test/test_sunau.py
@@ -5,10 +5,12 @@
 import sunau
 
 
-class SunauPCM8Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
+class SunauTest(audiotests.AudioWriteTests,
+                audiotests.AudioTestsWithSourceFile):
     module = sunau
+
+
+class SunauPCM8Test(SunauTest, unittest.TestCase):
     sndfilename = 'pluck-pcm8.au'
     sndfilenframes = 3307
     nchannels = 2
@@ -25,10 +27,7 @@
       """)
 
 
-class SunauPCM16Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = sunau
+class SunauPCM16Test(SunauTest, unittest.TestCase):
     sndfilename = 'pluck-pcm16.au'
     sndfilenframes = 3307
     nchannels = 2
@@ -47,10 +46,7 @@
       """)
 
 
-class SunauPCM32Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = sunau
+class SunauPCM32Test(SunauTest, unittest.TestCase):
     sndfilename = 'pluck-pcm32.au'
     sndfilenframes = 3307
     nchannels = 2
@@ -75,10 +71,7 @@
       """)
 
 
-class SunauULAWTest(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = sunau
+class SunauULAWTest(SunauTest, unittest.TestCase):
     sndfilename = 'pluck-ulaw.au'
     sndfilenframes = 3307
     nchannels = 2
diff --git a/Lib/test/test_wave.py b/Lib/test/test_wave.py
index 3e8ef51..9513df4 100644
--- a/Lib/test/test_wave.py
+++ b/Lib/test/test_wave.py
@@ -5,10 +5,15 @@
 import wave
 
 
-class WavePCM8Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
+class WaveTest(audiotests.AudioWriteTests,
+               audiotests.AudioTestsWithSourceFile):
     module = wave
+    test_unseekable_write = None
+    test_unseekable_overflowed_write = None
+    test_unseekable_incompleted_write = None
+
+
+class WavePCM8Test(WaveTest, unittest.TestCase):
     sndfilename = 'pluck-pcm8.wav'
     sndfilenframes = 3307
     nchannels = 2
@@ -25,10 +30,7 @@
       """)
 
 
-class WavePCM16Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = wave
+class WavePCM16Test(WaveTest, unittest.TestCase):
     sndfilename = 'pluck-pcm16.wav'
     sndfilenframes = 3307
     nchannels = 2
@@ -55,10 +57,7 @@
 
 
 
-class WavePCM24Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = wave
+class WavePCM24Test(WaveTest, unittest.TestCase):
     sndfilename = 'pluck-pcm24.wav'
     sndfilenframes = 3307
     nchannels = 2
@@ -85,10 +84,7 @@
         frames = audiotests.byteswap3(frames)
 
 
-class WavePCM32Test(audiotests.AudioWriteTests,
-        audiotests.AudioTestsWithSourceFile,
-        unittest.TestCase):
-    module = wave
+class WavePCM32Test(WaveTest, unittest.TestCase):
     sndfilename = 'pluck-pcm32.wav'
     sndfilenframes = 3307
     nchannels = 2
diff --git a/Misc/NEWS b/Misc/NEWS
index 327c3d7..3c94814 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -23,6 +23,9 @@
 Library
 -------
 
+- Issue #19623: Fixed writing to unseekable files in the aifc module.
+  Fixed writing 'ulaw' (lower case) compressed AIFC files.
+
 - Issue #17919: select.poll.register() again works with poll.POLLNVAL on AIX.
   Fixed integer overflow in the eventmask parameter.