You should really post code, even if it's wrong, to show some effort. However, this should get you started. Happy Holidays.
set targetFolder to (choose folder)
tell application "Finder" to delete (files of targetFolder whose name extension = "mp3")
or if you want to search recursively...
set targetFolder to (choose folder)
tell application "Finder" to delete (files of (entire contents of targetFolder) whose name extension = "mp3")