Excerpt from http://wiki.centos.org/TipsAndTricks/MultimediaOnCentOS
Step 1: Add RPMforge repository access for your CentOS system. (Required for the majority of multimedia packages.)
You can read the installation instructions at Installing RPMForge.
Step 2: Install the multimedia applications.
The following steps will use yum to install all the packages needed to have full multimedia support. Note there will be a lot of dependencies.
yum install compat-libstdc++-33 libdvdcss libdvdread libdvdplay libdvdnav lsdvd libquicktime
yum install flash-plugin mplayerplug-in mplayer mplayer-gui gstreamer-ffmpeg gstreamer-plugins-bad gstreamer-plugins-ugly
Step 3: Install the w32Codecs. (Required for xvid and other proprietary formats.)
rpm -ivh mplayer-codecs-20061022-1.i386.rpm
rpm -ivh mplayer-codecs-extra-20061022-1.i386.rpm
You should now have full multimedia and video support for most popular formats.