diff --git a/tests/gateway/conftest.py b/tests/gateway/conftest.py
index 978d634331a6..9cbc99fdd53f 100644
--- a/tests/gateway/conftest.py
+++ b/tests/gateway/conftest.py
@@ -184,6 +184,18 @@ def _ensure_discord_mock() -> None:
             self.description = description
     discord_mod.SelectOption = _FakeSelectOption
 
+    # AudioSource: real class so VoiceMixer(discord.AudioSource) can subclass
+    # it cleanly in tests.  MagicMock auto-attributes would make is_opus()
+    # return a Mock instead of False, breaking 9 TestVoiceMixerCore tests.
+    class _FakeAudioSource:
+        def is_opus(self):
+            return False
+        def read(self):
+            return b"\x00" * 3840  # one silent stereo s16 frame
+        def cleanup(self):
+            pass
+    discord_mod.AudioSource = _FakeAudioSource
+
     discord_mod.ui = SimpleNamespace(
         View=_FakeView,
         Select=_FakeSelect,