first of all, rhythmbox cannot play mp3 unless you have installed a patched version. mp3 support has been removed since Red Hat Linux 8.0 due to patenting and licencing issues.
* Is your soundcard configured? redhat-config-soundcard
* Are the audio driver kernel modules loaded? /sbin/lsmod
* Check the ownership and permissions of /dev/dsp. ll /dev/dsp
* Make sure you don't have a sound server (esd in GNOME, artsd in KDE) running, which blocks the device.
* Try a different player, e.g. xmms
-- get xmms-mp3 from ftp://ftp.uninett.no/pub/linux/Fedor.../rpm.livna.org