So I just thought of one possible reason to why some of the songs are out of key, and it has to do with chords. The Game Boy is only capable of playing 4 different notes at a time, with one of those notes being drums. So that leaves me with only 3 notes I can play at a time. I’m working off of piano sheet music, so that means one note for each hand, and if there is a third vocal track it gets the 3rd note. If there is no vocal track, then I typically give that third note to the right hand. So if I have a chord on the left hand, say an A and C chord, I’ll play an “average” and make it a B. So that would most likely be why it’s out of key. In fact, I often have to do this on the right hand as well.
My other option is to use simpler sheet music with no chords, but I don’t like using that. The songs come out sounding very flat and simple. It’s a much richer sound when I play more complex music. But if a song is too complex, then I don’t use it, it simply isn’t possible.