Can't mute YouTube layer

When I add a YouTube layer, ManyCam starts the audio. It is normal. But when I mute layer pushing the speaker icon on the upper right corner of that layer or by selecting “mute layer” in the popup menu, nothing happens. the sound is going on.

I reinstalled manycam, tried several (different) YouTube streams, used a different webcam for my own camera layer with no success. The noise is still going on.