As far as I know, "what sound" is any sound in general
That's what I thought. I didn't think she needed to sing for them to work; she can just pull a sound out of the air so to speak and transform it.
Also (and we've had this discussion before), she doesn't even seem to officially be a student at the school, much less received any training, and we've seen her in - what - 2 fights...she probably has no idea how to use those powers to fight or defend herself.