No volume when playing videos

HI, My son has an ipad and we only bought it at Christmas its the 7th generation. We cant hear any sound when playing you tube videos or videos we've taken ourselves. Even when we open things on the Internet no sound? The sound is right up its not muted. He can hear when he has his ear phones in? When we turn the volume up it plays the tone for volume perfectly fine so we no that the sound does work.