Disclaimer: I'm a new guy.
I have seen multiple videos on YouTube of people who have Gameboys with jacks attached that allow them to hook up to a mixer. (Reference: https://www.youtube.com/watch?v=CCTO9LRaox8 - I think he added some non-essential mods which I'm not interested in) If someone could link a tutorial (video preferred, but a text tutorial with pictures could do) that gives instruction on how to do this, that would be great. I have a GameBoy, GBA, and SP. I'm willing to do this mod on any of them.