So an MP4 is just a container, but you dont list the codec (i.e. the language) of the video file. Can you screen grab your export settings by any chance? MOST .mp4 files today use h.264 as the codec which is more or less the most universally compatible file nowadays, so Id be very surprised to here if you selected that. Of course, this is an audio codec issue from the sounds of it, not video, but usually audio settings are automatically set depending on which preset you selected. Definitely a bit odd, but please show us your export settings if you get a chance.
Hmmmmmm, those settings are pretty standard, assuming you haven't manually adjusted anything down in the Audio tab. They should work on 99.9% of modern devices. On the computers where it wont work, what player have you tried and have you tried a different player on those computers? This is pretty peculiar!
I wonder if you have audition ?
If you unlink your sound for entire sequence, nest it in another sequence by itself (copy paste, nest, so you keep your original stuff the way it is ), put that nested thing into audition, convert to wav 48mhz, etc., put that back into your sequence, link etc. Then export...how's that work for you ?
In other words maybe you can take you sound, make it wav instead of pcm or whatever it is, and try THAT on export it might be good on all computers, etc.
?? Wouldn't take long to test it out.