I have been unable to get my rear speakers to work with my SB Live. I am using the single digital out plug that comes w/ the card.
I also believe that I am using Alsa drivers, but I'm not 100% sure about that. I do know that in all of the mixers that I open there is no control for rear speakers, and the front two are the only ones playing sound. I'm not an expert w/ Linux, but I have been able to get pretty much everything working except for this.
I am using SuSE 8.2, and my speakers are just 4 speakers w/sub woofer. My SB Live is also the one that was out before the 5.1 came out. So neither speakers or sound card have 'point one' o them.