iOS 9.1 on iPad Air: No sound when I play videos

hello,
I updated my iPad Air to iOS 9.1 last week, everything is right until two days ago. When I tried to play a video, there was no sound even if I turned the volume up to the max. Then I restarted my iPad, it fixed the problem. But it happened again yesterday! Although restarting works, but I am sick of repeating it over and over again. I didn't drop my iPad or do anything to hurt it, and it works well on iOS 8 before. Does anyone meet the same problem? I don't know why this happen.