You will want to direct ALSA's oss emulation device 0 (/dev/dsp0, etal) to ALSA device 2, so that applications using the free/OSS API will produce their sounds on your nifty digital speaker system. To accomplish this mapping of free/OSS device 0 to ALSA device 2, add the following to your modules.conf file:
options snd-pcm-oss dsp_map=2
I put some more complete notes on this site: http://www.wlug.org.nz/NForce2Notes
One note, I am assuming here that the alsa driver provides the same devices for both the nforce2 and nforce3 motherboards. As I use an nforce2 based ABIT NF7-S (rev 2) I cannot confirm this assumption.