I wrote all of this and then took a closer look at the reference manual for Hitfilm Express and found that for MP4 files, AAC is the only audio format supported. I beg of FXHome that support for other audio types be added for MP4 files, such as MP3. Please, I just lost an absolute mountain of time in Handbraking footage.
This isn't the only one of my problems. First, .MTS files were being so slow when being played back in Hitfilm Express (newest version as of 4/16/2019) that they repeatedly crashed Hitfilm and so I used VLC Media Player to convert them to MP4, except now the audio track is different (now in MP3) and Hitfilm just doesn't seem to know it exists. I can play the file from my PC and it still has the audio. However, these are not the files I need the audio for, I need the audio in the paragraph below.
My display capture file was already MP4 but the audio of my voice that I needed was on the second track, which Hitfilm Express can't access. (Seriously?) So I used Handbrake (remembering that that's on my computer) and convert so that the second track is now the only track, and I find the audio to be again non-existent in Hitfilm, though again I can hear the audio just fine playing from my computer. In the original display capture footage, Hitfilm knows that the audio exists, it says the whole "stereo @48000hz" thing in the media tab. Writing this I am starting to realize that it is probably the fact that the audio track is now in MP3 format, but it took ages to convert this file, and it will be a real pain in the a** to have to reconvert this file again. If I have to convert again, however, please tell me which audio type should be used in .MP4 files.
PC Specs: Acer Aspire VX5
(Laptop) Intel Core i7 7700-HQ 2.8Ghz
(Laptop) Nvidia GTX 1050Ti
Hitfilm Express 12 (newest version as of 4/16/19)