--- a/dom/media/GraphDriver.h
+++ b/dom/media/GraphDriver.h
@@ -440,16 +440,22 @@ public:
     MOZ_ASSERT(OnThread());
     mAudioInput = nullptr;
   }
 
   AudioCallbackDriver* AsAudioCallbackDriver() override {
     return this;
   }
 
+  uint32_t OutputChannelCount()
+  {
+    MOZ_ASSERT(mOuputChannels != 0 && mOuputChannels <= 8);
+    return mOuputChannels;
+  }
+
   /* Enqueue a promise that is going to be resolved when a specific operation
    * occurs on the cubeb stream. */
   void EnqueueStreamAndPromiseForOperation(MediaStream* aStream,
                                          void* aPromise,
                                          dom::AudioContextOperation aOperation);
 
   /**
    * Whether the audio callback is processing. This is for asserting only.

--- a/dom/media/MediaStreamGraph.cpp
+++ b/dom/media/MediaStreamGraph.cpp
@@ -943,17 +943,18 @@ MediaStreamGraphImpl::PlayAudio(MediaStr
         output.ApplyVolume(volume);
       }
       t = end;
     }
     audioOutput.mLastTickWritten = offset;
 
     // Need unique id for stream & track - and we want it to match the inserter
     output.WriteTo(LATENCY_STREAM_ID(aStream, track->GetID()),
-                                     mMixer, AudioChannelCount(),
+                                     mMixer,
+                                     CurrentDriver()->AsAudioCallbackDriver()->OutputChannelCount(),
                                      mSampleRate);
   }
   return ticksWritten;
 }
 
 void
 MediaStreamGraphImpl::OpenAudioInputImpl(int aID,
                                          AudioDataListener *aListener)