iPhone, iOS 6, video from app (Gmail, Weibo, etc) no sound

Hi I have this problem since I upgraded iOS 6.

When I open a Youtube link in Gmail app, the youtube video starts to play, but there is no sound output. But if I click the open in safari button on the bottom right, and the video plays in Safari, the sound output is fine.

This happens to all videos in apps, like Weibo, DropBox, etc.

How to fix this?