--- a/dom/media/AudioConverter.cpp
+++ b/dom/media/AudioConverter.cpp
@@ -141,17 +141,18 @@ static inline int16_t clipTo15(int32_t a
 }
 
 size_t
 AudioConverter::DownmixAudio(void* aOut, const void* aIn, size_t aFrames) const
 {
   MOZ_ASSERT(mIn.Format() == AudioConfig::FORMAT_S16 ||
              mIn.Format() == AudioConfig::FORMAT_FLT);
   MOZ_ASSERT(mIn.Channels() >= mOut.Channels());
-  MOZ_ASSERT(mIn.Layout() == AudioConfig::ChannelLayout(mIn.Channels()),
+  MOZ_ASSERT(mIn.Layout() ==
+             AudioConfig::ChannelLayout::SMPTEDefault(mIn.Layout()),
              "Can only downmix input data in SMPTE layout");
   MOZ_ASSERT(mOut.Layout() == AudioConfig::ChannelLayout(2) ||
              mOut.Layout() == AudioConfig::ChannelLayout(1));
 
   uint32_t channels = mIn.Channels();
 
   if (channels == 1 && mOut.Channels() == 1) {
     if (aOut != aIn) {