I’m on 20.04 and downloaded Slack using snap. I downloaded Spotify via snap as well as my speakers work. Not sure why Slack isn’t working. I tried every single sound source.
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Is it possible there is another volume control setting for Slack application that was missed out? It is sometimes hidden deep in the Sound Settings under Applications.
I had a similar experience with Zoom for Linux. I could not hear a sound at all. The main system volume was all the way up. The app was connected to the device’s audio correctly. I reconnected a couple of times. Only to find out later on that there was yet another set of input/output controls for Zoom that was not so obvious. I only had to turn it up to hear sound during a call. By default it always seems to be at 0 when I join a call, which is weird.