you said you're playing on the gameboy? now that I think about it, i actually had a similar problem with my Game Boy Color, because the speaker had some water damage and couldn't play certain notes anymore. It's possible yours also has water damage and can't play the pitch/note like it used to. if you gba has a headphone jack, plug in headphones and see if the distortion goes away. If it goes away, then it's a speaker issue