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Taken from the Arch wiki
The default in Arch seems to be GStreamer, to try the GStreamer plugins do the following:
pacman -S gstreamer0.10-plugins
If you encounter problems that are not solved with this you can try switching to Xine (recommended):
pacman -S phonon-xine
Now choose Xine in:
System Settings > Multimedia > Backend (tab)
(it may have been autoselected after installing phonon-xine)