Automatic detection of not working audio device

[karlitschek]

It would improve the use experience if a user would get a warning before a call that the audio device is not working. Is it possible to read the audio signal and detect that it is silent? In this case there could be a warning next to the 'start call' button.

[nickvergessen]

We don't have media permissions before you start/join the call.
But after that I think it sounds very much useful, similar to #285

But I have no idea how to do this.