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
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