Ok so I downloaded video 9 and played it on my iphone 7. What I saw when the video first started playing was that it would actually pause about every 2 to 3 mins. The audio was not actually dropping but the video itself was pausing. Tapping the screen brought up the progress bar and you could see that the video was paused. Pressing play would resume the playback with the audio playing normally. this happened for about half of the video then it played the rest of the way without pausing.
I also tested 4 different MP4 files I have and did not see the issue at all. It seems the issue is mostly isolated to the source file. Looking at the video details it seems the file should be supported spec wise but it is encoded using Lavc51.9.0. the mp4 I have that are playing normally are encoded with x264. The issue with these files could be the encoding.
As an immediate work around you could use a media converter app and convert the file so it would be re-encoded using a different library and that may solve the issue.
