Problems with the Sound dropping out when Previewing Slide

In my Articulate 360 project I insert mp4 videos (created & produced in Camtasia) -1 video per slide.

When I am previewing the course prior to publishing - I have one slide where the audio narration starts out fine, but a minute or 2 into the video, the audio goes mute.

I checked my Camtasia project file - there is no issue with the audio there. 
I played the produced mp4 file on Windows Media Player - no issue there. 
I deleted the file from Articulate and then reinserted the video (in case it didn't load correctly the first time) - the issue remains. 
I have shut everything down, rebooted my computer, reopened Articulate & tried to preview again - the issue remains.   
The other slides in my project (all with inserted videos) are operating fine.   
How do I fix this issue!