diff --git a/dom/media/webaudio/AudioBlock.h b/dom/media/webaudio/AudioBlock.h
index 4d97bb47cae8..178ce8569dd3 100644
--- a/dom/media/webaudio/AudioBlock.h
+++ b/dom/media/webaudio/AudioBlock.h
@@ -46,7 +46,7 @@ public:
   using AudioChunk::SizeOfExcludingThisIfUnshared;
   using AudioChunk::SizeOfExcludingThis;
   // mDuration is not exposed.  Use GetDuration().
-  // mBuffer is not exposed.  Use SetBuffer().
+  // mBuffer is not exposed.  Use Get/SetBuffer().
   using AudioChunk::mChannelData;
   using AudioChunk::mVolume;
   using AudioChunk::mBufferFormat;
@@ -74,6 +74,7 @@ public:
     return static_cast<float*>(const_cast<void*>(mChannelData[aChannel]));
   }
 
+  ThreadSharedObject* GetBuffer() const { return mBuffer; }
   void SetBuffer(ThreadSharedObject* aNewBuffer);
   void SetNull(StreamTime aDuration) {
     MOZ_ASSERT(aDuration == WEBAUDIO_BLOCK_SIZE);
diff --git a/dom/media/webaudio/PannerNode.cpp b/dom/media/webaudio/PannerNode.cpp
index a95b5779ac30..11f76031bc70 100644
--- a/dom/media/webaudio/PannerNode.cpp
+++ b/dom/media/webaudio/PannerNode.cpp
@@ -462,9 +462,6 @@ PannerNodeEngine::EqualPowerPanningFunction(const AudioBlock& aInput,
       return;
     }
 
-    // The output of this node is always stereo, no matter what the inputs are.
-    aOutput->AllocateChannels(2);
-
     ComputeAzimuthAndElevation(position, azimuth, elevation);
     coneGain = ComputeConeGain(position, orientation);
 
@@ -508,9 +505,6 @@ PannerNodeEngine::EqualPowerPanningFunction(const AudioBlock& aInput,
     float orientationY[WEBAUDIO_BLOCK_SIZE];
     float orientationZ[WEBAUDIO_BLOCK_SIZE];
 
-    // The output of this node is always stereo, no matter what the inputs are.
-    aOutput->AllocateChannels(2);
-
     if (!mPositionX.HasSimpleValue()) {
       mPositionX.GetValuesAtTime(tick, positionX, WEBAUDIO_BLOCK_SIZE);
     } else {
diff --git a/dom/media/webaudio/PanningUtils.h b/dom/media/webaudio/PanningUtils.h
index a3be3f45e065..9c9db578db28 100644
--- a/dom/media/webaudio/PanningUtils.h
+++ b/dom/media/webaudio/PanningUtils.h
@@ -52,6 +52,8 @@ template<typename T, typename U>
 void ApplyStereoPanning(const AudioBlock& aInput, AudioBlock* aOutput,
                         T aGainL, T aGainR, U aOnLeft)
 {
+  aOutput->AllocateChannels(2);
+
   if (aInput.ChannelCount() == 1) {
     GainMonoToStereo(aInput, aOutput, aGainL, aGainR);
   } else {
diff --git a/dom/media/webaudio/StereoPannerNode.cpp b/dom/media/webaudio/StereoPannerNode.cpp
index 325a8854b0fc..339768c68498 100644
--- a/dom/media/webaudio/StereoPannerNode.cpp
+++ b/dom/media/webaudio/StereoPannerNode.cpp
@@ -92,16 +92,18 @@ public:
   void UpmixToStereoIfNeeded(const AudioBlock& aInput, AudioBlock* aOutput)
   {
     if (aInput.ChannelCount() == 2) {
-      const float* inputL = static_cast<const float*>(aInput.mChannelData[0]);
-      const float* inputR = static_cast<const float*>(aInput.mChannelData[1]);
-      float* outputL = aOutput->ChannelFloatsForWrite(0);
-      float* outputR = aOutput->ChannelFloatsForWrite(1);
-
-      AudioBlockCopyChannelWithScale(inputL, aInput.mVolume, outputL);
-      AudioBlockCopyChannelWithScale(inputR, aInput.mVolume, outputR);
+      *aOutput = aInput;
     } else {
       MOZ_ASSERT(aInput.ChannelCount() == 1);
-      GainMonoToStereo(aInput, aOutput, aInput.mVolume, aInput.mVolume);
+      aOutput->SetBuffer(aInput.GetBuffer());
+      aOutput->mChannelData.SetLength(2);
+      for (uint32_t i = 0; i < 2; ++i) {
+        aOutput->mChannelData[i] = aInput.ChannelData<float>()[0];
+      }
+      // 1/sqrt(2) multiplier is because StereoPanner up-mixing differs from
+      // input up-mixing.
+      aOutput->mVolume = M_SQRT1_2 * aInput.mVolume;
+      aOutput->mBufferFormat = AUDIO_FORMAT_FLOAT32;
     }
   }
 
@@ -119,7 +121,6 @@ public:
       // If input is silent, so is the output
       aOutput->SetNull(WEBAUDIO_BLOCK_SIZE);
     } else if (mPan.HasSimpleValue()) {
-      aOutput->AllocateChannels(2);
       float panning = mPan.GetValue();
       // If the panning is 0.0, we can simply copy the input to the
       // output with gain applied, up-mixing to stereo if needed.
@@ -138,7 +139,6 @@ public:
                            panning <= 0);
       }
     } else {
-      aOutput->AllocateChannels(2);
       float computedGain[2*WEBAUDIO_BLOCK_SIZE + 4];
       bool onLeft[WEBAUDIO_BLOCK_SIZE];