iMovie import of an MP4 will not play sound?

I had some VHSC videos digitized by Legacy Box and recently received my thumb drive back with the digital files. There were three tapes digitized. All three open and play fine in Quicktime however, when i import the MP4 file into iMovie and/or Final Cut Pro to edit, there is a problem. The video comes across fine on all three but of two of the three the audio is missing. How do I get sound working? I saw pretty much this same question asked before so I checked my original files and they are all the same (AAC, H.264). Why one of the three imports fine and the other two import the video but no audio is the mystery here. What to do to fix this?