Same problem here with 5.1 on a SB_XFi Xtreme Gamer
The best solution for me is to set the speakers to 5.1 in windows (I'm using xp pro) and to stereo speakers in rf2. It's not 5.1 surround sound but it does work rather nicely and the rear speakers don't cut out.
The settings below may or...
I also hope we get nascar type rules added to rf2 or at least the ability to add them. That alone would sale many copies of rf2, I would think.
It would be nice to hear something from isi on this :)