Videogram License (for video cassette, optical laser disc, home video product)

"Music licensing is the process by which songwriters, in theory, get paid for their work. In much the same way you don't own that copy of Doom or Windows, a purchaser of recorded music does not own the music, they own the media that music is stored on, and they have a limited right to use the music for themselves, so long as 'using' doesn't mean 'making unlicensed copies of' or 'broadcasting' the recorded work." - Wikipedia

You need to obtain a license because when you buy a music music track you are only paying for the right to listen to it yourself, not to distribute or play in the public domain.

In order to license a music track for a DVD, you would need first research who owns the copyright of the material you want to use, such as a record label like SonyBMG or Columbia records. You should also consult a music clearance and licensing expert to help with whatever you may need and get the process of licensing the track underway.

I found a website which has very cheap music licenses with a vast array of artists and music genres. www.beatpick.com
On this site I calculated that for a track on a DVD which had the following properties (Not in credits, background or source music, licensed for Europe only and a budget of less than 50,000 euros) it would cost £181.88 for the license.
This website encourages people to submit their own music and get paid through their licenses. Once you pay for the CD or other form of media, the tracks are yours to do with as you please.
For non-commercial usage, the site also has a Creative Commons License (http://www.beatpick.com/creative_commons.php), which allows the media to be used for non-commercial usage, such as students doing projects!

You could also use a Copyleft (http://en.wikipedia.org/wiki/Copyleft) license, which allows you to use and modify peoples creative work, but only as long as your work is freely available for others to do the same.

"Copyleft may also be characterized as a copyright licensing scheme in which an author surrenders some but not all rights under copyright law. Instead of allowing a work to fall completely into the public domain (where no copyright restrictions are imposed), copyleft allows an author to impose some but not all copyright restrictions on those who want to engage in activities that would otherwise be considered copyright infringement. Under copyleft, copyright infringement may be avoided if the would-be infringer perpetuates the same copyleft scheme." - Wikipedia

Another way to reduce the costs of music tracks, is to use Copyright/Royalty free music, where once you buy the CD you're free to use the tracks as much as you like in your productions. I found a website called AkmMusic (http://www.akmmusic.co.uk/) which supplies many big companies, as shown in the graphic I stole from their site.

The licence covers the following:
Television
Radio
Corporate Video/DVD
Training Video/DVD
Commercial Video/DVD
Point of Sale Video/ DVD
Amateur Video/DVD
Photographic Presentations
Message on Hold
Multimedia Presentations
Media Courses
Websites
Podcasts

Another way of course, is simply to make your own music tracks, using programs such as Soundtrack Pro. Or even record an unsigned band you know.
Personally I would go either with the Copyright Free license because of sheer simplicity of it, and you can re-use the tracks you are given multiple times. Otherwise if I am on a really tight budget, I would simply make my own tracks.