My wife's ML Soundwave's head no longer pops up in robot mode. I think the spring broke, but I'm not sure. Is there a way to fix it without ruining the MP3 play function?

 

you could try taking it apart to reset the spring. the mp3 function should be fine so long as you're careful not to damage the electronics)

 

If you take the head apart you can then pull it off the ball joint and then take the part with the ball joint out. I found that on mine the spring just wasn't pushing the head up enough so I carefully stretched it out a bit and put it together again. It's no actually better than new. I did have to put a tiny dab of superglue on the end of the spring to attach to the part with the ball joint as it was otherwise too fiddly to put back together.

 

I was surprised to find there was no ball joint on the head, that the spring just fits flat against the bottom of the head, and had slipped on its side. I super glued the spring to the head, and now it works fine.