There has been a report of problems with xine and vmix on Linux (here), not sure if this is the same issue, since you have problems without vmix too. Another (possibly unrelated) thing I've noticed: If you Ctrl-C osstest while it's playing, it will stop only after a bit more is played. On Linux this is instantaneous.
And, in fact, while testing xine (specifically, moving the progress bar around to skip to various points in the song), that machine locked up again. So mplayer is not the only trigger for the crash.
I think I'll switch to the ICH sound chip to see if I can get a kernel crash from that.
Adam
EDIT: It also happens on the intel hda sound chip on one of the two machines (the one that doesn't have the ich, obviously).