This is the quote from an option list on the Acadia.
DVD Entertainment System - 2nd-row, overhead DVD player, remote control, auxiliary input jacks, 2 wireless headphones, rear-seat audio controls and Advanced Bose 10 speaker 5.1 surround sound system, includes AM/FM stereo with CD player (deletes in-dash 6-disc CD changer) - - O
Navigation System - DVD-based, includes AM/FM stereo with separate CD slot, rear-seat audio controls and Bose 10-speaker sound system (deletes in-dash 6-disc CD changer)
I just answered a surround question in another thread, but I think this might be related. I could be wrong, but I think unlses you have a DVD audio disc, no normal store bought CD, nor AM/FM/XM encodes in 5.1 surround. So unless you're playing either a DVD movie or DVD audio disc (which of course would require a DVD drive, found only in car w/ the rear ent system), there's no way to get 5.1 surround audio into the stereo. I'm not sure about MP3 players. I do think AC3 and some other codecs allow 5.1 surround, so I'm not really sure there. The real question would be if they bothered to include a Dolby Digital 5.1 surround sound decoder in the radio without a DVD player. And that I don't know...