See:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/webaudio/DefaultAudioDestinationNode.cpp?dr=CSs&l=136

When the user sets the new channel count, the code does this:

...
destination_->Stop();
CreateDestination();
destination_->Start();
...

This triggers the re-construction of AudioRenderSink, FIFO, AudioBuses and WebThread class. Too much for the simple channel count change.

Consider refactoring this.