...just select a random text file which it will accept. Then go to HTML and change it to what you actually want the soundfile to be. Or just do the following.
Click on HTML button in FP, Look for the <head> tag and write this after it.
<bgsound src="music.mp3" loop="-1">
music.mp3 = the filename of your background sound
loop="-1" means the number of times you want it to loop. If you want it to loop forever, insert -1, for other values just write e.g. 1,2,3...