That's going to differ from guy to guy since every guy has their preferences. Personally, I consider music to be an incredibly attractive hobby for a girl to have. (Maybe that is because I like music myself and therefore like similar qualities, who knows). But if a girl can sing or play acoustic guitar, especially both at the same time, that is super attractive to me. Other instruments like drums or whatnot are really cool too. I don't meet a whole lot of female musicians in general so it is rare and it also means they have a common interest which is cool.