I don't think she does. She plays the triangle, and because she wants to get with Zii, she lied about playing bass. If you hadn't noticed, Sonya is not very bright. She may have thought that she could easily pick up how to play.I'm actually still surprised that it turns out that Sonya actually plays a base guitar.
Why else would she be "practicing" with a mini boombox, with her guitar not plugged in, and surprised that she broke a nail doing it?