Why is there no sound when I try to play YouTube clips? It was just fine earlier this evening.

A few hours earlier I listened to an old clip from Johnny Carson and everything was fine. Now even with the volume up to max, the picture plays but no sound (and yes I checked to make certain that I hadn’t accidentally pressed the slider down to nothing. )

I also did a reboot on the phone. It’s a Samsung Galaxy Core and it’s only a few months old.

Any ideas or suggestions?