Strange problem with music

[B.M.]

Hi list

I'm encountering a really strange problem with kdenlive - steps to reproduce
it:

Create new project (HD or, QHD, doesn't matter)
Add audio to project bin (e.g. flac 44100 Hz or wav 48000, doesn't matter)
Play audio: it works
Add it to an audio track: it works
Save file
Restart kdenlive
Open project file
Play audio clip in timeline or project bin: it doesn't work, i.e. I cannot
hear any sound
Remove audio clip from project file (and timeline)
Re-add it to project bin: it plays normally
Re-add it to the timeline: it plays

But note: if I add video clips (which include audio as well), they work as
expected. So in my main project (not this minimalistic test project), all
audio tracks work except the music audio track.

Version: 18.08.2 on Debian stable, but it doesn't matter - app image 17.12.0d
doesn't work either.

Any hint, where to look at?

Best regards and thanks a lot for your help,
Bernd