Probably operator error. 

Are your speakers on? Can you hear other audio presentations? Check the controls, often in the lower right of the screen.

Did you turn off the audio in the presentation? If so, the "speaker" will have a slash through it. Click the icon again.

I have a masters in electrical engineering  

I also have a Masters, but in Mathematics. I used to play a game called "WFF 'N Proof". Since this is a user forum with most answers given by other user, a variation of that on this site is a Well Formed Question.

What trial videos are you having problems with?