ALRIGHT. I've tested the 20/10 build and it works very well. Gambatte has perfect sound now! Thank you so much Aliaspider - this finally puts the GBC emulation issue on 3DS to rest once and for all.
So yeah, to everyone above, the 20/10 builds are fine. Didn't test 21/10 and 22/10 though, so the regression is in between 21 and 23.
Only one thing: I remember that tapping the upper part of the bottom screen used to turn it off, has that been disabled? It doesn't work anymore. I know I'm using a 2DS, IIRC the solution was making the bottom screen go dark.