Not every motherboard has 7.1 or they are really crappy.
If you spend the money on a real surround headset a sound card shouldnt be too much of a stretch.
The general rule with new builds is, get the BEST card you can buy(single) and if there is no better one get another.
Regarding 3 way SLI the performance boost of the third card is around 30-40% of the original card, overall it gives about 10-20% extra for the system. From 100fps you will jump to 120-130fps.