No sound in various apps on 19.10

I've recently installed 19.10 and am having trouble with sound in various applications. The system sounds work fine, which I guess means pulseaudio is up and running correctly. But Skype and Slack both have no audio and Facebook Messenger in Firefox prompts me to allow access to the camera and microphone, but then shows the error, "An error occurred while trying to get access to the camera and/or microphone: Hardware access aborted" when I allow it.

In Skype, I've had the echo test service work once but now it pops up "There was a problem with your speakers / microphone" and no ringing sounds appear when making calls or receiving them and the "Test Audio" button in the audio settings produces nothing. Skype doesn't show up in pavucontrol when doing the audio test.

In Slack, there is just no sound.

Some other apps, e.g., Spotify, do work.

Can anyone give me some pointers on debugging this?

Edit: Should note that this only fails with the "snap" versions of skype and slack from the Ubuntu software center; downloading the .deb from the website works fine.