Generally, audacious is installed, but no mp3 plug-in is installed by default.
Yum list audacious *
Displays all supported plug-ins of audacious.
[Root @ host ~] # Yum list audacious *
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* Rpmforge: fr2.rpmfind.net
Installed packages
Audacious. i386 1.4.6-1. el5.rf installed
Audacious-libs.i386 1.4.6-1. el5.rf installed
Audacious-plugins.i386 1.4.5-1. el5.rf installed
Available packages
Audacious-devel.i386 1.4.6-1. el5.rf rpmforge
Audacious-plugins-aac.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-alac.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-amidi.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-arts.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-esd.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-metronome.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-mms.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-mp3.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-tta.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-vortex.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-wavpack.i386 1.4.5-1. el5.rf rpmforge
Audacious-plugins-wma.i386 1.4.5-1. el5.rf rpmforge
Then,
Yum install audacious-plugins-mp3.i386
Yum install audacious-plugins-wma.i386