Welcome to Android Central! Are you sure the microphone isn't blocked while you're recording a video? Check to see if the mic has any debris or lint lodged in it. As far as I know, the One S has a main microphone at the bottom and a secondary noise cancelling mic at the top of the phone.
I've seen a few threads about a One S mic bug, but not much in terms of a solution. If cleaning the microphone port doesn't help, you might have to talk to HTC.