If you can play sounds on your system, I would suspect your problem is in XMMS configuration. Open XMMS, right click on xmms, and find the preferences. On the audio plugin tab make sure the output plugin is set according to the sound system you have installed. Most newer distros install ALSA. It could also be OSS ( Open sound system ), or even jack driver. This has to match your system, or you will get silence.
Also make sure you have the Mpeg 1/2/3 player input plugin installed for mp3 files.
Check what you have and post back if you have problems.