Why are MP3 files not noticeably compressed by the compression tools within Lion? I clicked the 'Compress xxx' option in the popup menu in order to compress a folder of MP3 files, but the result was a .zip file that is almost the same size as before compressing; it seemed to only archive those files.

How can I archive them and reduce their size greatly? 

Is there any free app that can do so?
As patrix mentioned in the comments, certain types of data (such as MP3, JPEG, MPEG) are not able to be compressed further because they are already compressed.

Compressing a bunch of MP3 files is not going to do much apart from bundling them together in one archive.