I got those errors during my experiments on FC4 with xmms and mp3 files... Especially when pressing stop button the xmms crashed w/ segmentation fault. It seemed to have something to do with sound system, with the plugin (alsa or oss, do not remember anymore which I had on then) or other settings. I could hear constantly static popping from my speakers, when they were connected to addon sound card (creative). Buildin card was disabled.
After battling with this, I decided to reinstall whole FC4.

Now, during this installation I left the buildin on and when it got to sound test, sounds worked on both creative and buildin. Now I do not get any segmentation faults on xmms.
I suggest that you check your xmms configuration for the mp3 plugin.