That's what I thought for the "wait until sound is finished" that doesn't stop the BGM. However I had already tried to replace it. It is possible to stop the BGM with the action "change background music of scene 'current scene' to '" (if you don't choose a music file, it will just stop playing the BGM, so it's the same result as the script).
Anyway, the problem is still the same, unfortunately
When a music is played as BGM it just stays and plays in loop. It's as if the action script didn't exist.
I'll make more tests tomorrow, I'm probably tired and making mistakes!