Do you think that it would be legal to:
a) Listen to a piece of music
b) Write the notes (a score) of the music for someone (ie, they commission you)

This would be obviously, only for music that you couldnt find the written score for.

Or is there some copyright protection that stops you from writing what you hear? Apologies for the lack of clarity - I have just bought some software that allows me to pretty accurately convert a music track into MIDI, and then can be exported as a score from Sibelius, or any other software of this kind.