On a stereo available cab (eg. Blast City), you just plug a RCA cable out of the Naomi into the cab.
On a mono only cab (eg. Astro City) using the Capcom or SEGA IO Board, you just plug the cable into the IO and adjust the levels directly on the IO and also your cab.
Alternatively, you can run the audio directly out of the Naomi into a set of stereo speakers (eg. Logitech)
I find the sound on the Atomiswave games to be super loud compared to native Naomi games, however they all need different levels of adjustment (eg. Marvel vs. Capcom is moderate in level, however Border Down is loud).