I've heard of that anyone good at music should be able to hear the thing when he reads the sheet music, or before playing it out etc.. I wonder how to develop this kind of skill(my listening skills are extremely poor by the way, and if I try imagine the shape of music beforehand, either it's wrong or worse, only blankness) I think i'm a bit slow at this. Although I started learning piano at quite a young age, but since it's a keyboard instrument so all you have to do is use your eyes doing some sheet reading then press the keys, and since no one is there to tell me to pay attention to what you hear, until now i'm still lost when all the people there say the only right way to music is to use ear...
if you want to compose / make up / invent what you are playing, then you'll probably want those hearing skills. but reading someone's exact transcription is a whole different way to experience music. very cool