Audio/sound not working on Xubuntu 18.04

I have a strange audio/sound problem on my Xubuntu 18.04 LTS box, for details see https://askubuntu.com/q/1211211/181442

The only application I am currently aware of that is unable to produce sound (after manually starting Pulseaudio) is Tor Browser 9.0.5.

To test whether this is an issue of the Firefox release Tor Browser is based on, namely 68.5.0esr (64-bit), I downloaded that and was able to play audio successfully (e.g. on Youtube).

I had a look at the Tor Logs, but they seem to only log events related to Tor connectivity, not browser-related events.

Trac:
Username: FriendFX