diff --git a/dom/media/VideoUtils.cpp b/dom/media/VideoUtils.cpp
index 04747d503378..298a9cf58066 100644
--- a/dom/media/VideoUtils.cpp
+++ b/dom/media/VideoUtils.cpp
@@ -167,6 +167,19 @@ void DownmixStereoToMono(mozilla::AudioDataValue* aBuffer,
   }
 }
 
+uint32_t DecideAudioPlaybackChannels(const AudioInfo& info)
+{
+  if (MediaPrefs::MonoAudio()) {
+    return 1;
+  }
+
+  if (MediaPrefs::AudioSinkForceStereo()) {
+    return 2;
+  }
+
+  return info.mChannels;
+}
+
 bool
 IsVideoContentType(const nsCString& aContentType)
 {
diff --git a/dom/media/VideoUtils.h b/dom/media/VideoUtils.h
index 80a4e3428152..14cbf56a6311 100644
--- a/dom/media/VideoUtils.h
+++ b/dom/media/VideoUtils.h
@@ -158,6 +158,10 @@ ScaleDisplayByAspectRatio(gfx::IntSize& aDisplay, float aAspectRatio);
 void DownmixStereoToMono(mozilla::AudioDataValue* aBuffer,
                          uint32_t aFrames);
 
+// Decide the number of playback channels according to the
+// given AudioInfo and the prefs that are being set.
+uint32_t DecideAudioPlaybackChannels(const AudioInfo& info);
+
 bool IsVideoContentType(const nsCString& aContentType);
 
 // Returns true if it's safe to use aPicture as the picture to be
diff --git a/dom/media/mediasink/AudioSink.cpp b/dom/media/mediasink/AudioSink.cpp
index dd687100ce5f..2ebd4ed787b0 100644
--- a/dom/media/mediasink/AudioSink.cpp
+++ b/dom/media/mediasink/AudioSink.cpp
@@ -64,12 +64,7 @@ AudioSink::AudioSink(AbstractThread* aThread,
   }
   MOZ_DIAGNOSTIC_ASSERT(mOutputRate, "output rate can't be 0.");
 
-  bool monoAudioEnabled = MediaPrefs::MonoAudio();
-
-  mOutputChannels =
-    monoAudioEnabled
-    ? 1
-    : (MediaPrefs::AudioSinkForceStereo() ? 2 : mInfo.mChannels);
+  mOutputChannels = DecideAudioPlaybackChannels(mInfo);
 }
 
 AudioSink::~AudioSink()
diff --git a/dom/media/platforms/agnostic/OpusDecoder.cpp b/dom/media/platforms/agnostic/OpusDecoder.cpp
index db420240487b..6489539e03db 100644
--- a/dom/media/platforms/agnostic/OpusDecoder.cpp
+++ b/dom/media/platforms/agnostic/OpusDecoder.cpp
@@ -95,6 +95,17 @@ OpusDataDecoder::Init()
                                                  mOpusParser->mCoupledStreams,
                                                  mMappingTable.Elements(),
                                                  &r);
+
+  // Opus has a special feature for stereo coding where it represent wide
+  // stereo channels by 180-degree out of phase. This improves quality, but
+  // needs to be disabled when the output is downmixed to mono. Playback number
+  // of channels are set in AudioSink, using the same method
+  // `DecideAudioPlaybackChannels()`, and triggers downmix if needed.
+  if (mOpusDecoder && mOpusParser->mChannels == 2 &&
+      DecideAudioPlaybackChannels(mInfo) == 1) {
+    opus_multistream_decoder_ctl(mOpusDecoder, OPUS_SET_PHASE_INVERSION_DISABLED(1));
+  }
+
   mSkip = mOpusParser->mPreSkip;
   mPaddingDiscarded = false;