I don't know of any utilities that can
repair audio files, just as there is no auto-picture repair utility. However, due to the structure of an aac file (and for iTunes they all come "wrapped" in an mp4 wrapper), it depends on where the damage is. Your aac file is actually comprised of around 60 'atoms' and only 1 of them ("mdat") is the audio. If the damage occurred in a non-mdat atom, you might be able to resurrect your m4a file.
See if you can open these songs in Quicktime Player, and export then as AIFF, and then re-import them using iTunes. You can also direct copy the audio track with a more comprehensive tool called MP4Box (part of an opensource tool called "gpac"). You can get a mp4box binary in a tool called ffmpegx. You'll have to choose "show package contents" and use it Terminal.app's command line:
/Supply/path/to/--->/ffmpegX.app/Contents/Resources/MP4Box -add ~/Desktop/Closer.m4a#audio -new ~/Desktop/try.m4a
(substitute the ~/Desktop paths with your files; note: you depending on your files, the '#audio' might need to be replaced with '#TrackID=1' or appropriate number).
I would advise you to try the mp4box way first (to minimize audio loss with multiple conversions) and listen to each song, checking each. If it fails, then I would try the Quicktime way. If that fails, I would go back to the source material. Whether you use Quicktime or mp4box, you'll loose the metadata. You'll need to re-do that.
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and I agree with dwb that iTunes didn't cause it. I would save things to cdr/dvdr before your data loss gets larger.