Nathan hit the nail right on the head in my opinion. Music and audio can completely change a consumers experience. The right music will keep them coming back for more, music that doesn't suit so well will put them off for life. Think of the amount of films you have watched that have a great soundtrack. Do you walk away remembering all of the dialogue or humming the theme? My guess it's the theme. Music helps people connect and should never be devalued.
I also wouldn't advocate the use of simply buying a piece of royalty free music to put behind the game. More often than not if you aren't a musician or composer yourself you aren't going to choose the best piece of music. Get the services of someone with experience in, even if it's just for them to provide a bit of advice on direction. It will pay dividends in the end product. And of course, as Nathan and others have said, offer them at least something in return. It's the polite thing to do.