I have two excellent applications (neither of them are iTunes) that can do this.
1) MP3 Trimmer
www.deepniner.net This can repair corrupted MP3 files without decoding/re-encoding them, and can now do that in batch mode. Free to try, $10.95 to keep...and IMO it's well worth it. It will also let you edit MP3's without decoding/re-encoding them (avoiding quality loss).
2) Max
sbooth.org Overall, an excellent ripping/encoding application, and it will let you re-encode existing MP3's (although you'll get quality loss from doing that). Completely free of charge, and IMO the best tool for MP3 encoding on Mac.
