I have this exact same problem on FreeBSD 6.1 i386 with an onboard Realtek AC97 and latest version of OSS:
I will run XMMS for a little while, and then a random combination of i/o devices freeze up: keyboard (no caps-lock light), mouse, screen, writing to hdd, and the audio itself, usually requiring a hard-restart.
Sometimes the sound skips and then XMMS freezes / audio stops, but I retain keyboard/mouse/gui control, but then when I try to save a file it will appear to have succeeded, but not exist upon restart (I am able to log off normally in this case). More commonly, the screen freezes, followed by loss of the keyboard, but I still have a [useless] cursor, and XMMS will continue playing as if nothing had happened until the end of the current track. Also: file explorer windows (various programs) will often freeze up when running XMMS; sometimes they come back after awhile and other times it develops into a full "crash".
Also: using aMSN causes the system to freeze or crash more frequently when XMMS is running (I think it may be because it's trying to play sounds from two programs at once, but that's not always the case, as it's also happened when just running XMMS, firefox, and thunderbird which uses the system speaker for newmail notification)
If I don't give the 'soundon' command and use XMMS, this problem doesn't seem to occur (but I can't be certain).