--- a/dom/media/AudioStream.cpp
+++ b/dom/media/AudioStream.cpp
@@ -349,20 +349,19 @@ AudioStream::Init(uint32_t aNumChannels,
 
   cubeb* cubebContext = CubebUtils::GetCubebContext();
   if (!cubebContext) {
     NS_WARNING("Can't get cubeb context!");
     CubebUtils::ReportCubebStreamInitFailure(true);
     return NS_ERROR_DOM_MEDIA_CUBEB_INITIALIZATION_ERR;
   }
 
-  cubeb_channel_layout layout;
-  int r = cubeb_get_preferred_channel_layout(cubebContext, &layout);
-  MOZ_ASSERT(r == CUBEB_OK || r == CUBEB_ERROR_NOT_SUPPORTED);
-  params.layout = (r == CUBEB_OK) ? layout : CUBEB_LAYOUT_UNDEFINED;
+  // The DecodedAudioDataSink forces mono or stereo for now.
+  params.layout = params.channels == 1 ? CUBEB_LAYOUT_MONO
+                                       : CUBEB_LAYOUT_STEREO;
 
   return OpenCubeb(cubebContext, params, startTime, CubebUtils::GetFirstStream());
 }
 
 nsresult
 AudioStream::OpenCubeb(cubeb* aContext, cubeb_stream_params& aParams,
                        TimeStamp aStartTime, bool aIsFirst)
 {

--- a/dom/media/GraphDriver.cpp
+++ b/dom/media/GraphDriver.cpp
@@ -634,20 +634,18 @@ AudioCallbackDriver::Init()
 
   output.channels = mGraphImpl->AudioChannelCount();
   if (AUDIO_OUTPUT_FORMAT == AUDIO_FORMAT_S16) {
     output.format = CUBEB_SAMPLE_S16NE;
   } else {
     output.format = CUBEB_SAMPLE_FLOAT32NE;
   }
 
-  cubeb_channel_layout layout;
-  int r = cubeb_get_preferred_channel_layout(cubebContext, &layout);
-  MOZ_ASSERT(r == CUBEB_OK || r == CUBEB_ERROR_NOT_SUPPORTED);
-  output.layout = (r == CUBEB_OK) ? layout : CUBEB_LAYOUT_UNDEFINED;
+  // Graphs are always stereo for now.
+  output.layout = CUBEB_LAYOUT_STEREO;
 
   Maybe<uint32_t> latencyPref = CubebUtils::GetCubebMSGLatencyInFrames();
   if (latencyPref) {
     latency_frames = latencyPref.value();
   } else {
     if (cubeb_get_min_latency(cubebContext, output, &latency_frames) != CUBEB_OK) {
       NS_WARNING("Could not get minimal latency from cubeb.");
       return false;