There are many tracks under the creative commons license that you can legally use for free. The licenses that you would be looking for are:
1 Attribution CC BY: this allows you to use or modify a work, commercially and noncommercially, as long as you credit the license holder or creator of the work.
2. Attribution-NoDerivs CC BY-ND: This license allows you to use a work, commercially and noncommercially, as long as you do not modify the work and credit the license holder or creator of the work. Basically, you use the song as is or not at all.
There is also an Attribution-ShareAlike CC BY-SA license that will allow you to use the work commercially, but you will have to license anything that uses the song, in this case, under the same license, meaning that others will be allowed to get everything in your slideshow and dvd's free of charge.
Link for more info:
http://creativecommons.org/licenses/