From ArchWiki

i18n
English
Ελληνικά
简体中文
Italiano

If you find that you cannot play common audio (such as MP3) or video files, you might not have the correct codecs installed to play them. Media players using the gstreamer backend (such as Totem) will be able to play most multimedia files after installing these codecs:

pacman -S codecs gstreamer0.10-bad gstreamer0.10-ugly gstreamer0.10-ffmpeg gstreamer0.10-mad gstreamer0.10-mpeg2dec

If you want to install ALL gstreamer codecs, you can you use the following command (assuming that you have awk installed):

pacman -Sy codecs `pacman -Ss gstreamer | grep -e '^extra/gstreamer0.10' | awk '{print $1}'`

You may still find that some files (especially Windows video files) won't play properly in Totem. VLC is a multipurpose media player which has many of its own codecs and can handle the tricky video files, as well as DVD movies with menus.

pacman -S vlc

Mplayer also plays many files. I have found that sometimes it will play things that won't play in VLC.

pacman -S mplayer

There also is a usefull mplayer plugin to integrate mplayer into browsers, install it with:

pacman -S mplayer-plugin