A bump for your topic and report of similar problem in google hangouts: the microphone is muted. The icons for Mute, Hang up and Show settings are invisible but labels appear on mouseover. Those functions seem to work; many combinations of settings tried (mute/unmute of course) but mic remains muted 😞 Microphone works fine in e.g. audacity.
MXLinux 18, Vivaldi 2.1.1337.51
Edit: the invisible buttons was caused by either uBlock Origin or Privacy Badger :oops: but disabling them did not solve microphone problem.
Edit: Weird, mic now works here in g-phone with settings I had tried before: Vivaldi microphone=ask; g-phone microphone=built in analog stereo. 🙄 Not necessary to disable extensions.
I'm switching to the 64-bit version of Vivaldi on my PC, to see for myself if I can notice any improvement or any new issues. I'm gonna stick with the 32-bit version on my tablet since it has a 32-bit OS anyway...