I think it's a perfectly good power for a deaf person. We are still discovering the extent to which our senses can evolve, especially when other senses are compensating for the lacking one. Transforming sound to light is a classic comic book power. Just wait until she learns how to harness that light into lasers and other hard-light constructs. The possibilities are endless!

I only wish Emma's character can evolve quicker than depicted. They were really dragging out her emo-ness.

I also wish Echo (black dude that died when Noah released him from Level 5 to kill Sylar) or Jesse (the hispanic dude from Veronica Mars who Peter possessed during Season 3) were still alive so Emma can have the perfect partner. (both the guys were sound manipulators)