It seems I can't get any sound from KDE on my Arch Linux installation.
Running the latest KDE 5 release, and while it seems that everything is working (and I have sound under Cinnamon) with KDE, while the sound control indicates enabled sound and the files obviously plays (according to graphic in e.g. Amarok) there is no sound at all. Setting the device to built-in and testing also produces no sound at all.

Any suggestions?
I've got phonon-gstreamer and phonon-vlc installed.

try killing pulseaudio
(#1 solution for all "can't hear a thing" problems)

There's either a problem w/ Phonon (do you get sound w/ mplayer or mpg123?) or the mixer settings - the volume of some alsa control is then likely at "0" - you could try "alsamixer" on the konsole or kmix (but w/ PA, you'll usually have only one entry there)

Will try. I just noticed that apart from working in Cinnamon, sound also works for root in KDE 5.
The default sound device is disabled for my regular user.

EDIT: The main problem seems to be that the built-in sound device is disabled in settings for some reason. It isn't for root, nor for my account when accessing System Settings from Cinnamon. It is equally disabled for test accounts I've created.

OK, looks like if I create a new account it will add 2 instances for the default built-in sound device. Disabling one of them makes it all work properly.
Why my original account had fubard sound is still unclear. If anyone has any ideas as to what might have happened to it ... Maybe something to do with an upgrade from KDE 4 to Plasma 5 kind of setup?
Else I'll just run with the new account.

I hope KDE developers follow this stuff. Anyway, given this is a perennial (multi-year) problem , and given the obscurity of the fix, I think it would be really nice if K-Mix were to have a diagnostic that ran at its start up and could say, "Once again, your sound has been disabled for your listening frustration. You might want to disable pulseaudio so that you will be able to listen again." and then explain exactly how to do that. It happens infrequently enough that I don't remember exactly how to do it and often enough that I find it pretty irritating. Best, of course, would be to not have competing sound management code each disabling the other when its turn comes along to be updated. This is pretty old now.