If you are listening on youtube and other sites make sure that the volume is not muted in the playing window
I mean in the you tube website

Hi, The problem that you are facing is a grave issue as it is related to internet browser itself. All you will have to remember the video website that you have been through recently and check where you have muted the sound. The situation that you are in currently is dealt like this only as muting sound on a website effects all other video or audio websites on your internet browser. However you can try resetting internet browser to manufacturer's default to check it further. Resetting Internet Explorer is easy. Open Internet Explorer > Tools Menu> Internet Options> Advanced tab> Reset. If you are using Firefox, open Start Menu>Run and then type "firefox -safe-mode" without quotes and reset firefox completely.
Hope that resolved your issue.
Cheers!