Firefox sound on Youtube is not more?

I have no idea why but for an hour after I turned off the laptop from the power saving mode and went on Youtube, the sound is gone. The video and the picture that's all normal, just stop without sound / sound. I already looked at the video if the volume is down but everything is normal. Then I restarted the laptop and reinstalled Firefox, but always the same problem.

I did not do anything and before the energy-saving mode Youtube was also quite normal. The other browsers with my other Youtube channels Youtube works fine and my offline videos stored on my laptop work fine. I was also not on Youtube Gestuched I looked after already.

Has clarified I looked at Google. It was the volume blender on Windows since Firefox was at 0 for all those who have the same problem.