5% of my songs have broken 'location' links - to random locations!

I have a Macbook with a Synology NAS hard drive, which I moved my iTunes library to recently. I used iTunes itself to make the moved (i.e. told iTunes I wanted the library in the NAS folder, told it to organise itself, and let it copy the files over).

While 95% of the files transferred correctly, some of them appear to have completely screwed-up file locations. One song below is linked to a folder containing university notes! iTunes doesn't even realise there's a problem - when I try to play it, it simply moves on to the next song and plays that instead (i.e. it doesn't even list it with an exclamation mark as a broken file). I first realised there was a problem when I tried to drag a couple of dozen files out of iTunes to put on a USB stick: 95% of them copied fine, but a few of them ended up copying 2GB folders across - because that's where the song was now linked to in iTunes.

The key issue seems to be that all of the 'broken' songs link to completely random, non-MP3, files. Another guy appears to have had the same problem some years ago, but other than him I can't find many references on the internet: http://apple.stackexchange.com/questions/66364/itunes-library-pointing-to-comple tely-wrong-files

Here's my question: how do I go through my iTunes database and identify all songs for which the location field is set to something other than an mp3 file (all my music is mp3), and - where it is wrongly set - correct it? I'm happy to pay for commercial software to do it.

Thanks in advance for any advice you can offer.