Trouble getting 5.1 to work in PC games

Are you confident that the sound chip will re-encode to DD or DTS? I don't know much about this MB but if it doesn't re-encode EAX (for example) to a DD multi channel stream, you're stuck at 2 channel sound. You may have to hook up the 6 channel analog outs.
-Trouble

No the problem is that the audio in DVDs is already encoded with Dolby but games are designed around analog multi channel audio not dolby. So in order to get dolby pro logic out of games either the game needs to support dolby out (which most don't) or the sound card needs to re-encode the audio to dolby. That feature is usually called Dolby Live or Dolby Master Studio.

I have the same situation as you and currently I just have my receiver apply a surround mode to the 2 channel output. I might get a Dobly Live supported sound card though.

You have another option:

If your receiver has analog inputs for 5.1 sound (RCA Jacks), you could get a breakout cable that connects to your motherboards standard minijack outputs for speakers (the way you'd connect standard PC speakers) and run those into the analog inputs of your receiver. Thats the way most folks have had to do it, since most sound card options dont support what you were trying to do above (take 5.1 game sounds and run them into your DD/DTS receiver via optical out).

connect the minijack outputs (all bajillion of them), or get a sound card with Dolby Digital Live encoding (DDL) from Auzentech, Blue Gears, or Turtle Beach and stick with optical out

X-Merdian is supported in Vista and does live encoding in DTS and DD.

Sadly, you may not know this, but if you had read the labeling of the P5N32-E SLI box, you would have known it's Sound Max chip hardware does indeed support Live DD encoding, however having read that would have only served to further confuse you as the drivers available as of 3 month ago didn't support that feature. Thats when most people gave up and bought a hardware sound card.