By default if you want to run some multimedia apps like music, song, video, flash player etc, you need to install the codecs and flashplayer plugins first. Here's the common codecs that recommended to install.
Open terminal then install with pacman:
#pacman -S a52dec faac faad2 flac jasper lame libdca libdv libmad libmpeg2 libtheora libvorbis libxv wavpack x264 xvidcore gstreamer0.10-plugins
$sudo pacman vlc audacious
Here's some snapshot.
Music player (audacious), runing mp3 file :
Flash player on the browser (mozilla firefox):
For another media like DVD, VCD, FLV player, etc, you can try it by yourself.
Enjoy multimedia on archlinux.