GStreamer is a pipeline-based multimedia framework written in the C programming language with the type system based on GObject.
GStreamer allows a programmer to create a variety of media-handling components, including simple audio playback, audio and video playback, recording, streaming and editing. The pipeline design serves as a base to create many types of multimedia applications such as video editors, streaming media broadcasters, and media players.
Designed to be cross-platform, it is known to work on Linux (x86, PowerPC and ARM), Solaris (Intel and SPARC), Mac OS X, Microsoft Windows and OS/400. GStreamer has bindings for programming-languages like Python, C++, Perl, GNU Guile and Ruby. GStreamer is free software, licensed under the GNU Lesser General Public License. (Source: Wikipedia)
Installing Gstreamer and plug-ins
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.