How's the sound on other videos, is it just this one that's the problem.
Are you using a desk top with external speakers with separate volume control (wife uses speakers for her ipad where it's speaker has become noisy, at times the jack plug may not connect or volume turned down/off) or you're on a notebook/laptop an built in speakers, and contact fault within the opening/closure of the screen. Then you are hearing sound, it's a little beyond me now as to the reason.
Cheers David