The best site I have seen in regards to mp3 licensing requirements is:

http://mp3licensing.com

They seem to indicate that commercial use requires a license:

http://mp3licensing.com/help/index.html#4
http://mp3licensing.com/help/index.html#5

I am not a lawyer, I have no knowledge if this site represents all possible patent claims related to mp3.  As always, I would advise people using technologies they suspect are patent protected to  seek legal advice to understand their own obligations based on their use and legal jurisdiction.
Mike

On Jun 21, 2010, at 8:56 AM, Tony Graziano wrote:

Since mod_shout encodes/decodes and audio file in the MP3 format, can
someone explain to me how this complies with licensing since MP3
encode/decode software is not "free".

Is the patent holder giving a free license to use the technology since
it is a open source package? Is another technology being used that
does not require licensing?