Yeah i had minor sound problems. I, too, thought it was my sound card. I have a chaintech 5.1 sound card, using 2.1 speakers. For the most part it was fine, but every once and a while the volume would go down for a few seconds. Also, during heavy fighting, sometimes i would get crackling.
As for the ending, I thought it was pretty easy. The only hard part is getting up that damn handicap ramp without getting wasted.