Gstreamer is an audio framework for linux. It can act as a go-between between a program and a sound device.
To play most media files using gstramer I recommend these steps:
1) Open a terminal interface (ex. terminal or konsole) and become root by typing:
2) Now type or copy this in to your terminal interface:
pacman -S gstreamer0.10 gstreamer0.10-good gstreamer0.10-bad gstreamer0.10-ugly gstreamer0.10-ffmpeg
Then install gstreamer0.10-amrnb  from AUR.
gstreamer-pulse should easily get you pulseaudio support for all applications using gstreamer audio at once:
pacman -S gstreamer0.10-pulse
"Whaawmp" is a slim GTK+ media-player like Totem (Gnome2 standard), that also uses gstreamer but has less dependencies - especially useful for Xfce that already uses gstreamer for its mixer. Whaawmp is available trough AUR as different versions, p.e.:
yaourt -S whaawmp-git
(git works best for me ATM, but that could change any time)
Amarok Gstreamer Plugin is (currently?) not available.