Music Licensing - Stock Music - Is this a sham???

I do not have any experience with these sites, but the idea is pretty standard now. Typically these libraries offer ad and film producers access to synchronization and other rights at very reasonable prices. Obviously this stuff does not become the sound track of an expensive production, but it lets them put something on the product that sounds like it is coming out of the car radio without having to pay a ton to clear stuff that actually comes out of car radios in reality. As often as not they are distributed as small cuts of a song, since it is not often necessary to play anything for a couple of minutes in the use to which producers put these fragments.
 
The promise is that you will make money on stuff you cannot sell otherwise, and the false promise is that you will be able to break in to major sound track work because someone will discover you from their twenty second purchase. The problem for making money is that they typically stock many of these so it is like listing your hand made doilies on Etsy. The other contributors to the library can be major competition if the site is willing to accept just anybody uploading anything. 
 
If they are charging you anything to contribute your stuff be very wary. If not, and the time cost of massaging your material into a format they require is negligible, you probably have little to lose, but probably also not a lot to gain.