Seomoz offers reasons why background music sucks on webpages:
Having music playing in the background can interrupt whatever is currently coming through the users speakers. Many people use their computers as media stations that play music and video. Having this suddenly interrupted is the equivalent of having a newspaper that jumps off the table and wraps around your head like a facehugger and starts laying headlines in your throat. There’s probably a better example to use when actually speaking to the client, but you get the idea: It’s forcing something down the users throat they didn’t ask for. Users expect web pages to contain useful information, not carry a tune.
A commenter elaborates:
I read somewhere that if you place a button that turns the music OFF, it will be the most clicked button on the page versus a button to turn the music ON which will receive very few if any clicks.
It’s a bad idea because people often surf the Internet when they should be working. The last thing you want is to click a link and have Hawaiian Music blaring loud enough for the boss to know that you’re planning your vacation instead of working. (Busted & No Sale)
You can’t control the volume of the end user. The end user might have been playing video games at full blast a while ago, but not now. They might have just put the baby to sleep and without warning… Crappy music suddenly starts playing at full blast and wakes the baby.