I found that a mp4 video plays without any audio in Moodle Mobile 3.2 on my iPhone 6s. The iPhone runs with iOS 10.2.1. I tested the problem with a video in a folder resource and with the video html5 tag which is generated by the Atto Media tool.
The course and the same video resource does not have any problem on an iPad Air 2 and on an iPad Pro 9.7. I got also no problem on an old iPad 2 running iOS 9.3.5 and Moodle Mobile 3.2.
Best regards, Ralf