videos lose sound after 30 seconds

All of the videos that I had uploaded from my iphone over a 2 year period have suddenly lost the audio after 30 seconds of video.

There are hundreds of videos of my kids and such, and previously they have played fine on my iMac. Recently after running an IOS update, when I watch the videos the audio stops at 30 seconds but the video continues (on ALL of them)!!!!

HELP!!!