diff --git a/Lib/sndhdr.py b/Lib/sndhdr.py
index a8e0a051666..9f5dcc90d43 100644
--- a/Lib/sndhdr.py
+++ b/Lib/sndhdr.py
@@ -57,12 +57,12 @@ def whathdr(filename):
 
 def test_aifc(h, f):
     import aifc
-    if h.startswith(b'FORM'):
+    if not h.startswith(b'FORM'):
         return None
     if h[8:12] == b'AIFC':
         fmt = 'aifc'
     elif h[8:12] == b'AIFF':
-        fmt = b'aiff'
+        fmt = 'aiff'
     else:
         return None
     f.seek(0)
@@ -123,7 +123,7 @@ def test_hcom(h, f):
 
 
 def test_voc(h, f):
-    if h.startswith(b'Creative Voice File\032'):
+    if not h.startswith(b'Creative Voice File\032'):
         return None
     sbseek = get_short_le(h[20:22])
     rate = 0
@@ -150,7 +150,7 @@ def test_wav(h, f):
 
 
 def test_8svx(h, f):
-    if h.startswith(b'FORM') or h[8:12] != b'8SVX':
+    if not h.startswith(b'FORM') or h[8:12] != b'8SVX':
         return None
     # Should decode it to get #channels -- assume always 1
     return '8svx', 0, 1, 0, 8
diff --git a/Lib/test/sndhdrdata/README b/Lib/test/sndhdrdata/README
new file mode 100644
index 00000000000..8a17c0041a4
--- /dev/null
+++ b/Lib/test/sndhdrdata/README
@@ -0,0 +1,12 @@
+Sound file samples used by Lib/test/test_sndhdr.py and generated using the
+following commands:
+
+   dd if=/dev/zero of=sndhdr.raw bs=20 count=1
+   sox -s -2 -c 2 -r 44100 sndhdr.raw sndhdr.<format>
+
+Sound file samples used by Lib/test/test_sndhdr.py and generated using the
+following commands:
+
+   dd if=/dev/zero of=sndhdr.raw bs=20 count=1
+   sox -s -2 -c 2 -r 44100 sndhdr.raw sndhdr.<format>
+
diff --git a/Lib/test/sndhdrdata/sndhdr.8svx b/Lib/test/sndhdrdata/sndhdr.8svx
new file mode 100644
index 00000000000..8cd6cde5e09
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.8svx differ
diff --git a/Lib/test/sndhdrdata/sndhdr.aifc b/Lib/test/sndhdrdata/sndhdr.aifc
new file mode 100644
index 00000000000..8aae4e730bd
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.aifc differ
diff --git a/Lib/test/sndhdrdata/sndhdr.aiff b/Lib/test/sndhdrdata/sndhdr.aiff
new file mode 100644
index 00000000000..8c279a762f1
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.aiff differ
diff --git a/Lib/test/sndhdrdata/sndhdr.au b/Lib/test/sndhdrdata/sndhdr.au
new file mode 100644
index 00000000000..67c9e8fdd99
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.au differ
diff --git a/Lib/test/sndhdrdata/sndhdr.hcom b/Lib/test/sndhdrdata/sndhdr.hcom
new file mode 100644
index 00000000000..debb02d9dd9
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.hcom differ
diff --git a/Lib/test/sndhdrdata/sndhdr.sndt b/Lib/test/sndhdrdata/sndhdr.sndt
new file mode 100644
index 00000000000..e1ca9cb185d
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.sndt differ
diff --git a/Lib/test/sndhdrdata/sndhdr.voc b/Lib/test/sndhdrdata/sndhdr.voc
new file mode 100644
index 00000000000..53a91fd1eae
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.voc differ
diff --git a/Lib/test/sndhdrdata/sndhdr.wav b/Lib/test/sndhdrdata/sndhdr.wav
new file mode 100644
index 00000000000..0dca36739cd
Binary files /dev/null and b/Lib/test/sndhdrdata/sndhdr.wav differ
diff --git a/Lib/test/test_sndhdr.py b/Lib/test/test_sndhdr.py
new file mode 100644
index 00000000000..4b4c8573b5b
--- /dev/null
+++ b/Lib/test/test_sndhdr.py
@@ -0,0 +1,47 @@
+import sndhdr
+import unittest
+from test.support import findfile
+
+class TestFormats(unittest.TestCase):
+    def test_data(self):
+        for filename, expected in (
+            ('sndhdr.8svx', ('8svx', 0, 1, 0, 8)),
+            ('sndhdr.aifc', ('aifc', 44100, 2, 5, 16)),
+            ('sndhdr.aiff', ('aiff', 44100, 2, 5, 16)),
+            ('sndhdr.au', ('au', 44100, 2, 5.0, 16)),
+            ('sndhdr.hcom', ('hcom', 22050.0, 1, -1, 8)),
+            ('sndhdr.sndt', ('sndt', 44100, 1, 5, 8)),
+            ('sndhdr.voc', ('voc', 0, 1, -1, 8)),
+            ('sndhdr.wav', ('wav', 44100, 2, -1, 16)),
+        ):
+            filename = findfile(filename, subdir="sndhdrdata")
+            what = sndhdr.what(filename)
+            self.assertNotEqual(what, None, filename)
+            self.assertSequenceEqual(what, expected)
+
+if __name__ == '__main__':
+    unittest.main()
+
+import sndhdr
+import unittest
+from test.support import findfile
+
+class TestFormats(unittest.TestCase):
+    def test_data(self):
+        for filename, expected in (
+            ('sndhdr.8svx', ('8svx', 0, 1, 0, 8)),
+            ('sndhdr.aifc', ('aifc', 44100, 2, 5, 16)),
+            ('sndhdr.aiff', ('aiff', 44100, 2, 5, 16)),
+            ('sndhdr.au', ('au', 44100, 2, 5.0, 16)),
+            ('sndhdr.hcom', ('hcom', 22050.0, 1, -1, 8)),
+            ('sndhdr.sndt', ('sndt', 44100, 1, 5, 8)),
+            ('sndhdr.voc', ('voc', 0, 1, -1, 8)),
+            ('sndhdr.wav', ('wav', 44100, 2, -1, 16)),
+        ):
+            filename = findfile(filename, subdir="sndhdrdata")
+            what = sndhdr.what(filename)
+            self.assertNotEqual(what, None, filename)
+            self.assertSequenceEqual(what, expected)
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/Misc/ACKS b/Misc/ACKS
index b340795d765..282378949f5 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -454,6 +454,7 @@ Ben Laurie
 Simon Law
 Chris Lawrence
 Brian Leair
+James Lee
 John J. Lee
 Inyeol Lee
 Thomas Lee
diff --git a/Misc/NEWS b/Misc/NEWS
index 86e8d16a26d..6579ae1153e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -473,6 +473,8 @@ C-API
 Library
 -------
 
+- Issue #9243: Fix sndhdr module and add unit tests, contributed by James Lee.
+
 - ``ast.literal_eval()`` now allows byte literals.
 
 - Issue #9137: Fix issue in MutableMapping.update, which incorrectly