As Bob said, there is a pause button, and you can hit that the second the music starts.

With some browser/machine/network/script-software combos, you can't. The machine may lock up trying to load the wherewithal to play the music and it might be several minutes before you can get it to respond to any user input at all. Or it may just lock up so thoroughly you have to force the browser to quit, or reboot the machine (the loading process may be so processor-intensive the pause button never appears on your screen). If you get the choice, you'll probably know which stuff works on your setup.