Strange bug caused by knotify which holds the sound device instead of allowing phonon to work (and so amarok).
$ lsof /dev/snd/pcmC0D0p COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME knotify4 3247 michi mem CHR 116,5 4182 /dev/snd/pcmC0D0p knotify4 3247 michi 14u CHR 116,5 0t0 4182 /dev/snd/pcmC0D0p
Solution: Disable the sound for notifications. Since KDE4 introduces new system settings instead of kcontrol, either type systemsettings (as user) in a shell or go to System -> System Settings.
Klick Notifications and go ot Player Settings, choose No Audio Output.