--- a/dom/media/AudioSegment.h
+++ b/dom/media/AudioSegment.h
@@ -216,20 +216,20 @@ struct AudioChunk {
     }
 
     // Memory in the array is owned by mBuffer.
     amount += mChannelData.ShallowSizeOfExcludingThis(aMallocSizeOf);
     return amount;
   }
 
   template<typename T>
-  const nsTArray<const T*>& ChannelData()
+  const nsTArray<const T*>& ChannelData() const
   {
     MOZ_ASSERT(AudioSampleTypeToFormat<T>::Format == mBufferFormat);
-    return *reinterpret_cast<nsTArray<const T*>*>(&mChannelData);
+    return *reinterpret_cast<const nsTArray<const T*>*>(&mChannelData);
   }
 
   PrincipalHandle GetPrincipalHandle() const { return mPrincipalHandle; }
 
   StreamTime mDuration; // in frames within the buffer
   RefPtr<ThreadSharedObject> mBuffer; // the buffer object whose lifetime is managed; null means data is all zeroes
   nsTArray<const void*> mChannelData; // one pointer per channel; empty if and only if mBuffer is null
   float mVolume; // volume multiplier to apply (1.0f if mBuffer is nonnull)