To fix this problem you need ManyCam (Free Edition). This is a virtual webcam + virtual microphone application to use Videos or Pictures like a Webcam.
The most games with a voice chat use the default input audio device. We can use ManyCam like a virtual mute microphone

1. Download & Install ManyCam https://download.manycam.com/

• Open ManyCam and remove the Webcam Source (only if you have a webcam)
  
• Click “Add a Video Source” and select “Blank image”

• Open ManyCam and add the Webcam in “Add a Video Source” (if not done yet)
  
• Click the little blue Speaker(on the right sidebar) of ManyCam and disable “Global Sound” to mute the audio of the webcam.

• Open the Windows Sound Setup and change the “Default” Audio Input Device to ManyCam Virtual Microphone
  
• Start talking to your headset / webcam to test the signal level of the microphone. It work if the level is always 0 (=no signal)

Check the Webcam & Audio Setup of ALL your Voice/Videochat Applications like TeamSpeak, Discord, Skype and Steam Audio Chat! These Applications should only use your headset & webcam and not ManyCam Webcam & Virtualmicrophone!