Code location:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/webaudio/MediaElementAudioSourceNode.cpp?q=MediaElementAudioSourceNode&sq=package:chromium&dr=CSs&l=126

Output(0).SetNumberOfChannels(number_of_channels) does attempt to change the channel count, but it might fail and the attempt might be handled later.

Consider re-arrange this process, so the request from the main thread is handled properly.