Hi,
I too am using a USB adapter and it works wonders.
"snd-usb-audio index=-2 to 0" is the only thing that needs changed to get it working.
If you're using mame4all-pi, you'll need the correct sample pack to go with the romset (0.37b5) in order to get fully working sound in a number of games. They belong in ~/RetroPie/roms/mame-samples.
Two others things I found that greatly improved sound using mame4all-pi (in dkong, for example) was the force-stereo option (I set it to yes, or 1, not sure of the exact 'syntax', in /opt/retropie/emulators/mame4all/mame.cfg, *I think*) and mildly overclocking the Pi itself (to 900 MHz, using raspi-config).
Even then, I find sound is still rather noisy for a number of roms. Must be due to the really old version number.
AdvMame and/or RetroArch and/or PiFBA may lead to a different experience. AdvMame, while theoretically slower, allows for newer roms. PiFBA also uses an updated, but smaller, romset. Neo Geo and a number of Capcom roms work better on PiFBA.