Hi [NAME], thanks for the tip. I already found the problem. The Windows service "Realtek Audio Universal Service" was not configured to start automatically. Now it works again. 
Hello, For several weeks now, my mute key on the keyboard does not work anymore.  - It does not change to white if pressed - Microphone is not muted if mute button is pressed Does anybody have an idea, how to solve this?
x15 R2, mute key does not work