Here is a quick guide to what I know about the laws on music:
IT IS ILLEGAL TO DISTRIBUTE ANY MUSIC FILE IN ANY FORMAT
(this includes MP3, M4A and "MP4")
IT IS LEGAL TO COPY MUSIC FROM A COMPACT DISC (CD) FOR OWN PERSONAL USE
IT IS POTENTIALLY (but not technically) ILLEGAL TO DOWNLOAD MUSIC
Effectively, it is your own choice to download music, and it is your responsibility if you are in possession of it unless you have an original, unedited copy.
THE ONLY LEGAL WAY TO DO THIS IS:
Either to ask the artist for permission, or actually buy the music. Unless, you are the artist, in which you can freely do whatever you want to do with it. I hear that iTunes Music Store is very popular.
Read more on the links above about Fair Use, or look on the links just below.
BBC Click, June 8th 2006: Copying CD music to a MP3 is legal
Microsoft Newsletter/RSS report: The benefits and risks of peer-to-peer file sharing