Panelists can't see or hear video playing

If they use the phone for their audio connection, they will not hear the audio for the video over the phone  If they have speakers on their computer, the audio will come through the speakers.

It could be a firewall issue or an internet connectivity issue for the Attendee that could keep the video from playing.

Since the video plays from the cloud directly to the Attendee's computer, the format of the video could make a difference.  Not all MP4s are created equal.  The Attendee's computer has to be able to play that particular version of MP4 video.  Different computer operating systems handle videos differently.

I always have a second computer running while I am presenting a GoToWebinar.  Often the video I start playing on the Presentation computer will start at a delay on the second computer and sometimes will not playback at all.  None of the reasons I gave earlier applied here.

The delay for me is a few seconds to several minutes.  Sometimes the video starts playing on the second computer before it starts playing on the Presentation computer.

If you have a short video, it could be just a delay issue rather then not playing at all.