Just be aware that some of the stuff floating around on the internet may be copyrighted - so if you put this stuff into a production that you're going to sell on a commercial basis or put on-air or whatever, you probably want to make sure that you have the rights to do this.
Be sure to read the usage terms for FreePlayMusic and make sure it really is free for what you are doing or be prepared to pay their licensing fee.
Also, note that the difference between public domain, copyright, and royality free.
Public domain means you can use it for whatever you want.
Copyright means you have to have the owners permission to use it for the specific purpose you intend.
Royality free means that the material is copyrighted, but the owner agrees to let you use it without paying any royalities. For example, there is some music (as well as sound effects) that you can purchase. Your are entitled to use it in commerial endeavors, but you cannot resell it. That means you cannot create a music CD using the music, but you can use it as background in an audio track where you read poetry or whatever.