[CentOS] Lenovo T420 internal microphone not working

Hello,

I have CentOS 7.1 installed on my Lenovo T420 Notebook. Now I will start to have some online meetings for a software project. In Bios the I/O-settings for camera and microphone are enabled. Camera is working (tested with cheese).

In the audio settings of Gnome perferences I see an internal microphone which doesn't produce any input. The microphone is also not disabled by the key on the keyboard.

Where can I take a look at?

Thank you in advance
Tim