When audio source initialization fails, e.g. if we hit the limit of concurrently allowed input sources, getUserMedia will report success and include a non functioning audio track with the returned media stream.
At the moment, the limit of audio tracks is 16, so one way to reproduce this is to open up 17 tabs that use audio input. The last one to open will seemingly succeed but won't get any audio (assuming there's nothing else already using audio input - if that's the case, failure will occur sooner).
Comment 1 by tommi@chromium.org
, Jan 8 2017