Why would the audio from .mp4 files be audible on some devices but not others?
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...