Skype for Business PPT embedded video not playing sound

SFB 2016 (16.0.4978.1000)

I have a PowerPoint presentation which contains a video (MP4) which has sound.

I have embedded the video as recommended by Microsoft
https://support.microsoft.com/en-us/office/play-a-video-in-a-skype-for-business-meeting-1bb30e51-d8f9-4ef6-b018-b170439e7488
I create a skype meeting, invite colleagues.  The meeting starts colleagues join in and I upload the ppt. 

They see the video in the slide, when they click on it the video starts but theres no audio.  If the click on the speaker icon it has a cross next to it which cant be removed.