Just use the onboard sound. Remember, most of the sounds are from the 70's, 80's & 90's where 3D, 4D, 2.1,3.1,5.1 or 7 channel surround and the likes wasnt even thought of.
I know I'll be contested with use of the onboard.
Also it shouldnt make MAME run any faster. Using a better video card will make MAME better since most of the action is on screen VS the sound! 64MB Ram video card should run most MAME games fairly well!
Hope this helps,

Fordman