It's important to realize that you can't just buy a song and use it without having the rights.
There are ways to get music free and legal, but you have to be aware of the situation.
Here are a couple of situations:
This is the video for my book "Mexican Slang 101". Over 30,000 views on Yahoo until they messed up, not over 30,000 on YouTube. Very viral item.
It has copyrighted songs in it, but there are permissions in place, allowing the rights holders to place ads on the bottom of the video. Note the little "buy on eMusic" button below it. No problemo.http://youtu.be/GA0BJX7kPTw
Here's a new one for "Way of the Weekend Warrior" using generic military music in the public domain. Very legal.http://www.youtube.com/watch?v=gnPqTU3y ... AtjOa4mdLA
This one for "Imaginary Lines" uses "Royalty Free" music I purchased from a service. There are lots of places to buy music that you can legally use. Prices vary wildly.http://www.youtube.com/watch?v=JO7DxOXs ... AtjOa4mdLA
This one for "Engines of Desire" uses music by my friend Duane Pesice, with whom I've done several videos.http://www.youtube.com/watch?v=O1OkJF0D ... AtjOa4mdLA
A note on that: it's good to have buddies who are musicians. And original music is actually better for your book than some pop chart single. But you can find people who do music bu searching music sites. And will probably find them delighted to provide a tune for the credit. It's fun for them to have a video of their music.
Let me show you this one for a poem from my as yet unpublished collection "The Great Man". The idea here was that the video is a promo for the artist, Will Jacques, the musician (Duane again) and myself. Something to keep in mind.http://www.youtube.com/watch?v=2_BLTqJ7 ... AtjOa4mdLA
Another thing to keep in mind-- a major advantage in doing your own video is being creative, not making it look like all those other damned slideshows.
Lastly, here's another approach, the soundtrack for this one for one of my authors was generated by a program that generates music. Much sounds lame, if you mess with it you might get something that works for you.
I let the title of this one be a surprise. http://youtu.be/wB_v_HgzO5Y
I have some stuff on music and sound tracks, with lists of sources in my ebook manual on the subject. http://adorobooks.com/works/tutvideo.php
That demo video on that page, by the way, has more generated music tracks on it, show a little variety.http://youtu.be/wB_v_HgzO5Y