Pigster14,
Whether you have to pay a per-use royalty or residual to a music copyright owner or not depends on where and from whom you get the music.
Some companies (mine, for example) produce what is sometimes called 'rights-managed' product. These music products are licensed and sold independently of performing rights organizations such as ASCAP and BMI by means of a rights management agreement directly between the purchaser and the copyright owner.
Such products are similar in some ways to purchasing a CD containing clip art. Much of this that you might see in your local computer store comes only with the right to include the images in your business advertising, but not the right to include the images in salable product or product packaging, which is not what you would need in this case. There is, however, also higher-end clip art, fonts, etc., that you can purchase specifically for use in saleable product, where you pay a flat one-time fee for a nonexclusive right to use the material as often as you choose in marketable products. The same thing exists with music and sound libraries.
Essentially, you buy a license to distribute the music in the library as you see fit, subject to certain limitations--for example, within only a specified region and/or for only a specified period of time (as some software developers sometimes distribute their products). Sometimes there are no such time or space limitations (as is common with many high-end orchestral sound sample libraries) but in any case such a license agreement generally requires that you must include a substantial contribution of your own material in addition to the copyrighted material within any product you sell containing the music--i.e., you can't simply resell the music by itself on a CD or downloadable as a music file or in any way that effectively amounts to simply reselling the music to others for use as you are using it.
Radio stations, for example, purchase CDs of jingles which they may then use as components of advertising commercials to which they add voiceover talent. They're not free to simply duplicate the CD and sell it to other radio stations, but they may use one particular jingle in an unlimited number of radio commercials. Some videogame companies get sound effects and musical themes this way. Corporations sometimes get music for promotional or training videos this way, too. Small budget films and documentary or educational videos sometimes get soundtrack music this way.
If you're trying to evoke a sentimental memory to go with someone's family vacation snapshots turned into a multimedia slideshow, the high-amped garage rendition of your neighbor's kids' medly of original 3-string 'wannabe-hits' may not be the soundtrack you're looking for. Whatever you're looking for, though, it's likely available in the way I describe.
(On the other hand, you may just happen to personally know the next 'big' thing in pop music and help to put them on the map with your slideshows, so take my last comment with a grain of salt.)
--torandson
