• start Steam
  
• start CS GO
  
• open Steamoverlay ingame (standard with Shift + Tab)
  
• open Webbrowser in Steamoverlay
  
• open a website that plays sound (for example a music video on Youtube)
  
• tab out of Steam (standard with Alt + Tab)

Then you click with the right mousebutton on the speaker symbol in your task bar...








and open the volume mixer.









Then search for the "Steam Client Webhelper" and drag down the bar to lower the volume or mute it completely.











That´s it!

You must not take this long way with the Webbrowser. When you are on a server and an advertisment is running you can go directly to your volume mixer and the Steam Client Webhelper should appear there.

This method also mutes or lowers the sound of the ingame-radio. So when you are on a server and want to hear the radio you need to enable the sound of the Steam Client Webhelper again.