Got this idea from a study that cows produce more milk when listening to music they like. Listening to music wouldn't turn her into a cowgirl, but would probably affect her when she's already in that form.
I ended up going for a Disgaea-y attempt, with colored lineart mixed with lineweights. Originally she had earbuds to fit in her cow ears (and that's probably what she uses anyway), but it was a little hard to identify that she was listening to music instead if just sleeping, so I used headphones. They clamp down on her ears but she can still hear.
Here's some possible melodies Mattie could be listening to: