This is an excerpt taken from the Linux Kernel 2.2.x Changes file.
RealPlayer
Current releases of Real Player 5.0 depend on a bug in the sound sub-system which is no longer there. Consequently, they don't work. Real is aware of the problem and should have an updated version of the software available shortly. In the mean time, you can always try backing up your copy of rvplayer, and then editing it by:
dd if=/dev/zero of=rvplayer bs=1 count=1 seek=657586 conv=notrunc
dd if=/dev/zero of=rvplayer bs=1 count=1 seek=665986 conv=notrunc
If you're lucky, you'll then have sound.... You may also need to edit it with
dd if=/dev/zero of=rvplayer bs=1 count=1 seek=702554 conv=notrunc
as well.

Alternately, download rpopen from http://onramp.i2k.com/~jeffd/rpopen/ and pre-load it before you run rvplayer (it's a shared object which blocks rvplayer from doing the NONBLOCKing open of /dev/dsp).