Alright! Thanks to Bulldogs here and Temujin|Futurelooks on IRC I've fixed things!
Long story short: It was a driver issue. Installing the latest beta sound drivers for the P6T Deluxe v2 fixed it (even though I think I don't have the v2, just the Deluxe).
Long story long: After trying Realtek 2.25, Realtek 2.25 beta, Realtek 2.27, P6T Deluxe latest drivers I decided to try the P6T Deluxe V2 drivers. They appeared to be more recent and I didn't think that the sound chip changed. If it did, well, the V2 drivers appear to be backwards compatable. The version that worked is labelled AD2008 Audio Driver 6.10.02.6570.