It's not like an audio in, like a mic input, it means that the DMG can take sound from an external sound chip if it's inside a GB cartdige, like the VR6/7 expansion chips for the NES.
edit:
Specifically for LSDJ, nothing. It's designed to be used on re-programmable cartridges, which don't have expansion chips. If there was either a) a re-programmable cart with an expansion chip or b) Johan started manufacturing carts with an expansion chip, then it would have a use with LSDJ, but only if he wrote a version specifically for writing with that chip.
Last edited by Saskrotch (May 10, 2012 7:26 pm)