Thank you Jan Gruuthuse,
I found and installed a package of Avidemux 2.6.4 (that's what my package manager says, the "about" function gives me no version number).
Now I have some audio devices to choose, and "Alsa default" works as far as video and audio run synchronously when I play the video in avidemux. I can also drag the cursor to a certain position and then cut and save the video, and the saved version is ok.
However, whenever a video plays and I click the "stop" button, avidemux will freeze and no longer react to anything, I can only kill the process. This only happens when some audio device is selected. When I set audio device to "dummy", I can stop and restart the video with no problems.
I also cannot choose any "audio output" in the left pane. I don't know yet if I really need this feature but something seems to be wrong here.
Do you have any idea what I could do?
PS. is there a "show my own posts" function in this forum? It's sometimes not easy to find them...