Why would the audio from .mp4 files be audible on some devices but not others? I don't know if this question is best suited for Signal Processing, Sound Design, or Video Production. If someone knows for sure that can do so, please migrate this question to the best forum for it (I'm guessing "Signal Processing" for now).

I have posted some mp4 files on the Internet, both on YouTube and on a dedicated facebook page. Most people have no problem with seeing the video and hearing the audio emanating from these files.

Two people, though, have complained (or mentioned, anyway) that they cannot hear the audio. One of them was using a Windows desktop, and the other an iPhone.

What are the reasons why some may not hear audio, while others can - when they can hear audio otherwise on their device?

The files in question (half a dozen of them) are here.

For a direct link to one particular one, you can try this.

Or, from YouTube, here's one.

Is it anything on my end that I can "fix," either in-place or locally and then re-upload? It'd be hard to test when I had accomplished my goal of making them audible to all, as they work for me already...
When such is the case, it's most probably a broken flash or incompatible browser for those who cannot hear the sound of your videos. However if those two can hear all other internet videos but only yours, then this is really a complex issue. Considering the fact that there are so many different platforms, browsers, devices, drivers and media formats it will be really hard to find out why...

for example I can not even run the videos you have uploaded on my windows 8.1 laptop with firefox browser. It's probably because I'm using a flash blocker to prevent unwanted streaming of flash videos whenever a page is loaded. Also firefox is known to casue erratic behaviour for flash videos through its latest versions...