This is a follow-on to bug 884365, which uses direct notification of AppendToTrack for audio to avoid any delay imposed by MediaStream and avoid needing possible resampling to avoid clock drift (see bug 908834 and bug 818822). If a stream is attached that's not a LocalMediaStream, MediaPipeline should look for normal MediaStream output instead of the direct notifications. This will enable gUM plus WebAudio (and other scenarios that are similar) to work. Note that this will have some added latency, and without the resampling bugs fixed may cause an ever-increasing delay or periodic underruns/glitches.
I believe this is fixed now (see MediaPipeline.cpp:825) if it was ever actually broken.