Need HELP pelase with Sound of Gecko Browser

Hello,
I try to mute any outgoing sound from my desktop c# application using Gecko Browser. I tried also mute application sound and also Browser, but sometimes it is working and sometimes it is not.
So how could I proceed please? Any senior c# developper if he take few minutes of his time and help me with this I will really appreciate, as I tried also microsoft stuckoverflow but they didn't help and they told it is not IE so they can not or they don't liek to help.
It is so frustrating during 2 weeks about and my project is so hugh that I can not come back to IE as it will take another 2 month to rewrite it back.

Thank you for any help or suggestion.