This could handle a separate thread, but anyway:
There are a bunch of tools that can help you rename files. pfrank and ant renamer come to mind. They are both free, and can help you quickly and easily rename files.
pfrank -
http://www3.telus.net/pfrank/
ant renamer -
http://www.antp.be/software/renamer
pfrank has a greater learning curve than ant does. Since we're just renaming extensions, I would go ahead and just use ant renamer.
Click on "Add folders..."
Choose a folder (it helps if the entire folder contains the files that are all going to have the same extension, like mp3. You may want to organize them first)
Click on Actions
Choose Change extension
Enter the extension - example: mp3
Click on Go, and thats it!
(I'm pretty sure there is another thread about renaming files on here somewhere)
Edit: -
http://www.tfproject.org/tfp/showthread.php?t=109127