Akumajou Densetsu with extra sound channels - how?

[ChurchOfSolipsism]

So I'm using the RGB Dual (which is powered by Recalbox) with an RPi4, and it's a pretty nifty machine. Problem is, I want to enjoy Castlevania III with the extra music channels provided by the Japanese cart, but with the emulator included in Recalbox the sound is for some reason fucked (truncated western version plays fine though). So, how do I get to play this gem with proper music?

[ChurchOfSolipsism]

So in case anybody cares, you can switch the emulators in Recalbox on a game to game basis (highlight a certain game in your game list, press start, then edit, and pick a different/ the second included emulator), which will give you proper VCR6-sound.