The speaker does play the test sound of KMix settings for all users.
It works well when playing movies with Firefox too. However this works only with one user. Other users don’t get any sound.
Also invoking speaker-test in a terminal window works only for one user. The other users don’t get any sound.
All in Audio group?? Sometimes needed.
You are absolutely right. Sound on the mainboard never needed that. The USB sound in the Logitech speakers does need it.
I added users to group audio and logged them in again. This did not work however. Then I reinstalled USB sound using YAST. Now there is USB sound for all users. 
Many thanks for helping!
You do have to log the user out and back in if doing the change under their account. ie the group change does not take place until the next login of the user.
I had already entered “groups” in their account and had found no change. Thus as described above I logged them in again. This implies logging out before doing so. Logging in again did not do the trick. Only reinstalling USB sound via YAST changed the situation. This tidied up the display at Settings > Multimedia > Audio by properly graying out all of three mainbord sound entries and enabling USB sound.
Hmm maybe you broke something when poking things trying to get it to work for all.
Any how good that it is working
I can’t tell what was wrong with the in initial installation. I just plugged in the speakers and fired up YAST > Hardware > Sound. I added USB sound and deactivated onboard sound. Then I finished by clicking OK until done, no more tinkering.
Any how good that it is working
Yes, I was quite optimistic, because all users had sound. Later on it turned out, that there is a nasty side effect: Switching from graphical login of user A to user B results in grabbing of inactive user’s A **kwin **process 100% cpu load. It never happens the other way. User B’s kwin never exhibits this rogue behaviour. I was logged in as user A when installing USB sound. When user B is playing a movie Firefox keeps playing the movie upon switching to user A and vice versa. Switching users never caused any trouble when using onboard sound. Any thoughts?
Found out what went wrong. When booting the machine everything was back to the situation in post #1. Boot messages were hinting to sound server kill:
2014-12-21T13:00:01.933607+01:00 hofkirchen systemd[1339]: Stopped target Sound Card.
2014-12-21T13:00:01.939192+01:00 hofkirchen systemd[1339]: Starting Default.
2014-12-21T13:00:01.940004+01:00 hofkirchen systemd[1339]: Reached target Default.
2014-12-21T13:00:01.940571+01:00 hofkirchen systemd[1339]: Startup finished in 46ms.
I found out that openSUSE had installed both alsa and pulseaudio. The startup script killed pulseaudio replacing it by alsa. This worked fine for onboard sound but failed when adding USB sound. I deinstalled pulseaudio and rebooted. Everything works fine after startup without further tinkering.
alsa and pulse are not mutually exclusive. If you can live with out pulse fine.
I deleted the server only and kept the libraries. Actually alsa and pulseaudio are collaborating. Rogue kwin got fixed by rebuilding the akonadi database.