No sound on Ubuntu 18.04.2
I use the AppImage on Ubuntu 18.04.2 LTS
After I have started MuseScore I first have to do "Edit -> Preferences -> I/O -> Restart Audio and MIDI Devices" to be able to hear any sound. Either for play notes when editing or playback of a loaded score. This is the case for version 3.0 and 3.1. Not in in the old 2.3.2.
Interestingly that the nightly builds do not show this behavior. The last one which I tested is MuseScoreNightly-201906201513-master-1c53795-x86_64.AppImage. Here I can here the sound right after startup. No restart of audio devices needed.