Basically because it's too expensive
and apparently the on-board sound card that PCS have is very good anyway, so I'm just sticking with that. That also supports 5.1, and if I get enough money in the future, I'll buy a 5.1 speaker system. I'll mostly be using the speakers for music anyway (a load of bass for my dubstep) and I'll be using my headset for games.
I changed the spec a bit more, and here's the new one:
Not a huge change, but it knocked a bit of the price, and I have a better power supply, just in-case I want to add more to it in the future, and ends up requiring more power.