Weird Speaker Issue, Only works when opened then closed.

My macbook speakers only work when you start the video, then close the laptop then open it. I have tried restarting it, resetting it completely, reinstalling it and opening the laptop to unplug the speaker then plug it back in. The sound stops working when a new video starts playing. I dont know why its doing this but when i watch a youtube video then a ad comes on and the sound goes off again, i have to either close the laptop or put it into sleep mode.

Thanks

Alfie