I can't hear the sound on my iphone

I just made a video on my iphone 4 and when I played it I can't hear the sound. I checked the ringer as well as the volume also. But it doesn't seem to work. But when I played the video that I made a few days ago that works fine and I can hear it also but the one I just made I can only see the video but can't hear any sound.

This happened to me before. Switch it off for 5 min, then on again?