Sound comes on and off through the jack plug

I have a strange problem, when using the jack cable, evreytime the sound of the video lowers, the sound fully cuts off on the speaker and turns back on when the video sound goes up. For example people are having a conversation in a movie, then they stop speaking, the sound cuts off and the conversation starts again and it takes a second or 2 for the speaker to start emitting sound again. 

What can I do about that ?

I have 2 speakers and it does it with both. I tried changing the cable, trieded it on diferent computers and nothing changes.

I'm running it on Mac and can't do firmware upgrades. I also reset the seakers and still no change.

Thank you for your help.