All of a sudden MuseScore started "not responding" all the time and stopped playing note sounds when editing

I tried reinstalling MuseScore: the problem did not go away.
I checked:
1) mixer - everything is fine. nothing is muted, nothing is soloed
2) in the settings "Play notes when editing" is ticked
3) in general sounds are present when I press "PLAY"
Just last week everything was normal. No bugs, no missing sounds.
That all changed when I plugged a midi keyboard and headphones with a sound card into the laptop. I was then, on the contrary, trying to remove those sounds when editing, as they were interfering with my transcription.