FirebrandX seems to be on a quest, lately, to extract “audiophile” sound quality out as many systems as possible.
He wrote a detailed guide on his website about restoring flawless audio on the NeoGeo AES notoriously bad 3-3 and 3-4 board revisions. The process, being fairly simple, involves soldering a few ceramic capacitors on the underside of the board, adding an electrolytic capacitor (only found on MVS boards) and swapping a couple of already present polyester capacitors on the main-board.
The guide has been recently updated with the use of higher-standard parts that FirebrandX applied himself on his own test-subject.
Check out the full guide for a list of requirements, a lossless audio demo and other details that owners of 3-3 and 3-4 AES boards will surely find useful: http://www.firebrandx.com/aes3-3-3-4audiofix.html
While you’re at it, be sure to check the RGB bypass mod for the same revisions, also written by FbX: http://www.firebrandx.com/aes3-3-3-4rgbbypass.html