Actually, they don't seem to mind. they sleep during the time I did it before. I don't want something to hurt them though, either.
I do not think the noise will injure your chicks.
And even if it did hurt their ears, I've read that chickens can regrow cells inside their ears, so hearing loss is not permanent for them the way it is for people.
And you already know they act fine, so I don't think you need to worry about the music causing any problems for your chicks.