Syndrome: I have a second sound card (Creative X-Fi Champion) besides the on-board HDA Intel soundcard. Before upgrading the Amarok to version 2.0.2 (along with upgrade of Ubuntu to 9.04) everything was okay with Amarok. Afterwards however the Amarok refuses to play any music file, and warns something like 'too many errors to play the playlist'. Starting from command line suggests Phonon(sound API for KDE) has problem to access the Creative card (resource busy) and uses 'default' - which does not give any sound output.
Solution: Some posts found via Google suggests it might help to install GStreamer backend for phonon, therefore I went to Synaptic package management, and installed phonon-backend-gstreamer, phonon-backend-xine, and several other packages of phonon (like phnon-dbg, libphonon-dev, etc). Now Amarok rocks again:) Experiment showed that phonon-backend-xine is essential, since sound disappears again once it is uninstalled.