Hmm.. .net 4.5 is included with 8.1, but you may want to see if you can do a repair install. I don't think that's it but could be worth a shot. When you checked your sound drivers did you go through windows or the manufacturer of the sound card? If not the manufacturer please do so.

Finally, if you could run a dxdiag report and either paste it here or pm me I'll look over it tonight and our logs to see what else could cause it.

Edit: fist was faster!