Firstly, check if the drivers are installed properly. Go to Start > Run > devmgmt.msc. If there is a sound device in yellow, right-click it and choose Update Driver or install the normal driver if you have one. Secondly, if you already have the audio installed normally, go to system tray and locate the icon that manages your audio settings (not the Windows one). Switch from 5.1 to Stereo or Headphones and then reload the YouTube pageб and it should work.
This issue is known among users with RealTek and VIA sound cards.