User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36

Steps to reproduce:

go here: https://launchinteractive.com.au/firefox/ in firefox
click play
toggle audio buttons

Actual results:

volume of audio increases as target video 1 and 2 are unmuted
source plays no audio when targets are muted

Expected results:

I would expect source to control the audio volume and it shouldn't matter if targets are muted.

When all targets of mozCaptureStream() stream become muted, the original (source) media element does not produce any sound. It does not matter if the targets will be muted through the buttons or directly from the media element, the result is the same. Is that expected?

This is one of the reasons we haven't been able to drop the moz- prefix of this API.