--- a/dom/media/AudioStream.cpp
+++ b/dom/media/AudioStream.cpp
@@ -124,17 +124,16 @@ AudioStream::AudioStream(DataSource& aSo
   , mInRate(0)
   , mOutRate(0)
   , mChannels(0)
   , mOutChannels(0)
   , mAudioClock(this)
   , mTimeStretcher(nullptr)
   , mDumpFile(nullptr)
   , mState(INITIALIZED)
-  , mIsMonoAudioEnabled(gfxPrefs::MonoAudio())
   , mDataSource(aSource)
 {
 }
 
 AudioStream::~AudioStream()
 {
   LOG("deleted, state %d", mState);
   MOZ_ASSERT(mState == SHUTDOWN && !mCubebStream,
@@ -326,17 +325,17 @@ AudioStream::Init(uint32_t aNumChannels,
   if (!CubebUtils::GetCubebContext()) {
     return NS_ERROR_FAILURE;
   }
 
   MOZ_LOG(gAudioStreamLog, LogLevel::Debug,
     ("%s  channels: %d, rate: %d for %p", __FUNCTION__, aNumChannels, aRate, this));
   mInRate = mOutRate = aRate;
   mChannels = aNumChannels;
-  mOutChannels = mIsMonoAudioEnabled ? 1 : aNumChannels;
+  mOutChannels = aNumChannels;
 
   mDumpFile = OpenDumpFile(this);
 
   cubeb_stream_params params;
   params.rate = aRate;
   params.channels = mOutChannels;
 #if defined(__ANDROID__)
 #if defined(MOZ_B2G)
@@ -348,21 +347,16 @@ AudioStream::Init(uint32_t aNumChannels,
   if (params.stream_type == CUBEB_STREAM_TYPE_MAX) {
     return NS_ERROR_INVALID_ARG;
   }
 #endif
 
   params.format = ToCubebFormat<AUDIO_OUTPUT_FORMAT>::value;
   mAudioClock.Init();
 
-  if (mIsMonoAudioEnabled) {
-    AudioConfig inConfig(mChannels, mInRate);
-    AudioConfig outConfig(mOutChannels, mOutRate);
-    mAudioConverter = MakeUnique<AudioConverter>(inConfig, outConfig);
-  }
   return OpenCubeb(params);
 }
 
 // This code used to live inside AudioStream::Init(), but on Mac (others?)
 // it has been known to take 300-800 (or even 8500) ms to execute(!)
 nsresult
 AudioStream::OpenCubeb(cubeb_stream_params &aParams)
 {
@@ -547,31 +541,27 @@ AudioStream::GetPositionInFramesUnlocked
 bool
 AudioStream::IsPaused()
 {
   MonitorAutoLock mon(mMonitor);
   return mState == STOPPED;
 }
 
 bool
-AudioStream::Downmix(Chunk* aChunk)
+AudioStream::IsValidAudioFormat(Chunk* aChunk)
 {
   if (aChunk->Rate() != mInRate) {
     LOGW("mismatched sample %u, mInRate=%u", aChunk->Rate(), mInRate);
     return false;
   }
 
   if (aChunk->Channels() > 8) {
     return false;
   }
 
-  if (mAudioConverter) {
-    mAudioConverter->Process(aChunk->GetWritable(), aChunk->Frames());
-  }
-
   return true;
 }
 
 void
 AudioStream::GetUnprocessed(AudioBufferWriter& aWriter)
 {
   mMonitor.AssertCurrentThreadOwns();
 
@@ -590,20 +580,20 @@ AudioStream::GetUnprocessed(AudioBufferW
   }
 
   while (aWriter.Available() > 0) {
     UniquePtr<Chunk> c = mDataSource.PopFrames(aWriter.Available());
     if (c->Frames() == 0) {
       break;
     }
     MOZ_ASSERT(c->Frames() <= aWriter.Available());
-    if (Downmix(c.get())) {
+    if (IsValidAudioFormat(c.get())) {
       aWriter.Write(c->Data(), c->Frames());
     } else {
-      // Write silence if downmixing fails.
+      // Write silence if invalid format.
       aWriter.WriteZeros(c->Frames());
     }
   }
 }
 
 void
 AudioStream::GetTimeStretched(AudioBufferWriter& aWriter)
 {
@@ -618,20 +608,20 @@ AudioStream::GetTimeStretched(AudioBuffe
   uint32_t toPopFrames = ceil(aWriter.Available() * playbackRate);
 
   while (mTimeStretcher->numSamples() < aWriter.Available()) {
     UniquePtr<Chunk> c = mDataSource.PopFrames(toPopFrames);
     if (c->Frames() == 0) {
       break;
     }
     MOZ_ASSERT(c->Frames() <= toPopFrames);
-    if (Downmix(c.get())) {
+    if (IsValidAudioFormat(c.get())) {
       mTimeStretcher->putSamples(c->Data(), c->Frames());
     } else {
-      // Write silence if downmixing fails.
+      // Write silence if invalid format.
       AutoTArray<AudioDataValue, 1000> buf;
       buf.SetLength(mOutChannels * c->Frames());
       memset(buf.Elements(), 0, buf.Length() * sizeof(AudioDataValue));
       mTimeStretcher->putSamples(buf.Elements(), c->Frames());
     }
   }
 
   auto timeStretcher = mTimeStretcher;

--- a/dom/media/AudioStream.h
+++ b/dom/media/AudioStream.h
@@ -328,18 +328,19 @@ private:
   }
 
 
   long DataCallback(void* aBuffer, long aFrames);
   void StateCallback(cubeb_state aState);
 
   nsresult EnsureTimeStretcherInitializedUnlocked();
 
-  // Return true if downmixing succeeds otherwise false.
-  bool Downmix(Chunk* aChunk);
+  // Return true if audio frames are valid (correct sampling rate and valid
+  // channel count) otherwise false.
+  bool IsValidAudioFormat(Chunk* aChunk);
 
   void GetUnprocessed(AudioBufferWriter& aWriter);
   void GetTimeStretched(AudioBufferWriter& aWriter);
 
   void StartUnlocked();
 
   // The monitor is held to protect all access to member variables.
   Monitor mMonitor;
@@ -369,19 +370,15 @@ private:
     STOPPED,     // Stopped by a call to Pause().
     DRAINED,     // StateCallback has indicated that the drain is complete.
     ERRORED,     // Stream disabled due to an internal error.
     SHUTDOWN     // Shutdown has been called
   };
 
   StreamState mState;
   bool mIsFirst;
-  // Get this value from the preference, if true, we would downmix the stereo.
-  bool mIsMonoAudioEnabled;
 
   DataSource& mDataSource;
-
-  UniquePtr<AudioConverter> mAudioConverter;
 };
 
 } // namespace mozilla
 
 #endif

--- a/dom/media/mediasink/DecodedAudioDataSink.cpp
+++ b/dom/media/mediasink/DecodedAudioDataSink.cpp
@@ -62,18 +62,20 @@ DecodedAudioDataSink::DecodedAudioDataSi
     // content provider want change from those rates mid-stream.
     mOutputRate = mInfo.mRate;
   } else {
     // We will resample all data to match cubeb's preferred sampling rate.
     mOutputRate = AudioStream::GetPreferredRate();
   }
   MOZ_DIAGNOSTIC_ASSERT(mOutputRate, "output rate can't be 0.");
 
-  mOutputChannels = mInfo.mChannels > 2 && gfxPrefs::AudioSinkForceStereo()
-                      ? 2 : mInfo.mChannels;
+  bool monoAudioEnabled = gfxPrefs::MonoAudio();
+
+  mOutputChannels = monoAudioEnabled
+    ? 1 : (gfxPrefs::AudioSinkForceStereo() ? 2 : mInfo.mChannels);
 }
 
 DecodedAudioDataSink::~DecodedAudioDataSink()
 {
 }
 
 RefPtr<GenericPromise>
 DecodedAudioDataSink::Init(const PlaybackParams& aParams)