I think it's a great idea! Adding music can enhance the mood if its done correctly. My only problem would be what would happen if the music would refresh for every page. Some things I would recommend:
1) Add a mute button to the sound. Some people like listening to their own music on the computer while reading online, so if the only way to stop the music is to exit the page or turn the volume off on the computer entirely that would piss off a ton of people.
2) Avoid using wordy songs. Wordy music tends to draw away from the text and distracts the reader. Use only if it completely complements the text.
3) Keep in mind that music you like may sound utterly horrible to someone else, thus defeating the purpose of setting a tone or an atmosphere. There have been many times where I would navigate to a website only to cringe at the sound of a barrage of horrible banging and shouting noises only to realize that this was supposed to be "music." Subjectivity is fine, just remember that there are some forms of music that more people can relate to more easily.