I have the suggestion that I always give in these situations: First decide what you want and design everything and THEN start to build it. There are several things you have to consider, for example if a user should be able to vote for as many songs as they like, if they should be able to vote for the same song more than once and if they should be forced to be logged in to vote. The answers to these (and more) questions will determine how you should continue.
However you will do the voting I suggest that you use 2 fields in the song-table, number_votes and total_points. Everytime someone votes number_votes is increased by 1 and total_points with the point of the vote. When you want the mean just divide total_point by number_votes.
