No playback, Linux
This problem may have solved itself, but the diagnostic method might be worth noting. On the HowTo page it might be worth suggesting starting from a terminal to get diagnostic messages. And in my case that's all it took to fix it (I think).
In my case playback usually works normally, but starting yesterday it stopped working. The cause is undetermined, but it may be related to Audacity.
Here's what happened:
* I started it in the usual way from the menu. No sound. The HowTo page (https://musescore.org/en/node/96811) did not solve the problem. (I didn't replace drivers, but restarting devices from the Musescore I/O Preferences menu didn't help).
- I copied the command from the menu and restarted it from the terminal. Sound. Go figure. I have no idea why. I made NO changes.
/home//ProgramFiles-downloads/MuseScore-3.4.2-x86_64.AppImage musescore --style /home//.musescore/Styles/MyDefault.mss
QApplication: invalid style override passed, ignoring it.
I think the QApplication message is irrelevant.
Edit>Preferences>I/O shows Jack Audio selected (in both working and nonworking state).
Ubuntu 16.04 (fully patched and up to date)