Yes, but that doesn't explain how to fix it. The signal is clipping (slight distinction from too much gain, because too much gain causes clipping) if you notice, one note is mostly clear, but the more notes played, the worse it sounds. He’s adding all the tones together, and not re-normalizing them before his addition. Which i suppose could be thought of as “not adjusting individual gain down” before adding signals together. Also filtering really would work wonders if he's just generating pure tones (sine waves).