It's just crossed my mind that the reason iTunes couldn't play it is because it was converted to a format quality that iTunes isn't catered to play perhaps. I'm not sure what iTunes limits are. But perhaps it's been converted to a Sample Size like 4 bit which iTunes doesn't have the ability to convert to & perhaps not the ability to play.

I am aware many audiobooks are converted to very low bit rates so perhaps this person reduced the sample size also which maybe iTunes can't handle. Converting to WAV should fix that issue with any luck! If not, we'll need to check what it's sample size is just to be sure in case it also needs to be converted. By default WAV's sample size is 16 bit