If the music plays before the page fully loads, this keep the attention of the customer focused on your site. if the customer alwasy has to wait a long time for the page to load before the music play, they may be tempted to leave your sight. in this case speed is an portant factor. when you change pages and you have to wait for the song to start playing after the page loads, this becomes a distraction and disatifaction to the listen ears of you customers. Don't let your customers loose interest. keppe them engaged.