Google - in the utube app ,andriod, the video stays in place at some point but the audio goes on.y?

Because the video and audio streams are seperate and synced during play back. Because the audio stream is a loaded faster than the video stream (it's usually a much smaller size to upload) and there is a buffer upload to ensure smooth playback then the buffer for the audio is larger than the buffer for the video. As a result the video may freeze during playback because the video stream has "paused" while the audio continues on, playing from the buffer.