I recently switched to Linux and I'm having an issue with audio when calling people on Skype and Discord. My audio works for both recording and playing sounds but if i'm calling my friends on Skype or Discord they can hear me but I can't hear any sounds. I tried force restarting pulseaudio and keeping the settings to 'Default Device' on Skype/Discord but they don't work.
Even I faced this problem but I was not using Ubuntu instead I was using Windows only.
I guess there's a third-party app messing up with voice settings or maybe there might have been an issue while installing.
I would highly recommend you test it with the Discord Web Version and if you still can't hear people on discord then here's are some more methods which you can follow. Hope it would be helpful.
Cheers, Sandra Smith