Actually MP4s can be just audio files as well, though as you say they are primarily found as video files.
If one of the bad files is small enough, post it somewhere where we can download it and test it. Maybe one of us will be able to see what is wrong. If you don't have a small sample, try trimming one down, testing if it imports, and if not post it somewhere for us.
Alternately, edit one of the bad files and save it to a new name as an MP4, the see if the edited file imports.
PS: CountryBumkin, you should use the Sourceforge link for MediaInfo as that source has no attached trial or other software. The link you used has been criticised as having adware installed with it that is hard to remove.