So... I am still getting very troublesome audio issues on my end. Not all games, mind you, and not all the time... but often enough, that it is not ignorable.
Just started Sonic Wings 2, and a handful of samples just "screach" in your ear. Similar to MSlug3 I said earlier. Sometimes, resetting and reseating the cart helps. But... I can't play like this. None of my original games have this (so I'd say I would rule out my NeoGeo in this, but I have to do more extensive tests, i.e. find one of my games, that has this issue and then not being able to replicate it with the flash cart). Now I will try a different SD card, as the one I used for the MultiAES was just one I still had lying around. Now I'll try a "known good" one.
I'll post a video on the weekend, showing my issues.