Problems with sound on mp4s?

[AC Question]

SO I downloaded a couple mp4s onto my Samsung. But they only seem to play with sound when I open them in MX app (but not in the standard video app). And even more frustratingly, when I try to share the video to instagram, the sound is missing there as well.
By downloading the MX player, I can at least watch an mp4 with sound through that app. But why have I lost sound on the video player? and how do I get this video to instagram on my phone WITH sound?