Did you ever resolve this? I have a similar situation with a High Speed (System 11), although I never had music. When I press the sound test button, I get a repetitive two tone sound output (sounds like a siren), which from what I think I understand to mean that U21 ROM is bad too. I had the same issue with the battery but didn't look like much damage, so I'm trying to figure out if I need to replace the whole sound board or just locate a U21 ROM.