Onboard sound these days can be quite reasonable and will often be better than older dedicated solutions. Having said this though I cant say I know anything about what sound is being used for x1000.
As for me wondering why a south bridge is neeed it appears PA6T has no intergrated pci controllers which probably plays a large part.
SB600 gives you PATA, SATA, Audio, USB, PCI, RTC, LPC Bus and more. All of which the PA6T doesn't have.
