I had a similar problem with Fedora and resolved it by reinstalling
PulseAudio.You could also just remove PA and go with ALSA.You could also
play around with the backend in the System Settings, under Hardware,
Multimedia under Phonon on the Backend tab. You can choose between Xine and
Gstreamer. Mine is set for Xine which seems to work better. If you don't
have both installed then you can install it. I mention this because when PA
was worse than it is now, if you can believe it, the solution for multimedia
not playing in Ubuntu was to install Totem-xine which removes Gstreamer as
the default.It is worth a shot. Also it is worth installing VLC for
multimedia playback.