I've had this strange problem with BeOS on my
original Athlon (one of those big, slot loading types)
for quite some time, and it really is not cool. About
75% of the time, the system clock ticks twice as fast
as it should -- that is something I could live with.
BUT, it also screws up playback of any media files
(movies, audio, etc). It essentially ends the movie
before its over. For example, the movie plays just
fine, but the little status bar moves across the screen
twice as fast as it should, ending playback of the file
way ahead of time.

I've asked around about this before, and someone
else told me that only the Athlon 750 MHz models
have this problem. I haven't touched BeOS in quite
some time (simply because of this problem), but if
there have been any fixes, etc., I'd really like to know
about it, as the BeOS is certainly my favorite!

Actually the bios is doing that. There are some patches at bebits for some cards.

I haven't been able to find any such patches... can
anyone point me in the right direction?

Thanks a million for the help!

If your card isn't mentioned in one of the apps then either just try something similar and hope or maybe ask one of the authors to add support for your card/chip.
There are some problems with some players and some types of files.

Might ask on beshare.