Why isn't my iMac switching the input sources?

GoetzSaCebu

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I use a 2017 iMac and OSX is BigSur now.

The internal mic plus then a ATR2100USB and a HD Webcam with build in mic.
ATR and Webcam are connected using the same USB3 Hub.

Now I made a video showing how I switch the input source on my iMac and then knock on the 3 different input sources.
Its blurry because the webcam didnt really get the focus but you can see the VU meter on the app good enough.

What makes me wonder is, that even I switch the sources and my knocking shows it correct in the app running in my iMac, the sound is totally different - showing that always only the internal mic is recording. So I wonder WHY is that and WHAT can I do to correct that error!

The video is in german but you can see me knocking the sources and the VU bar and then compare that to what you really hear.
Does anyone can explain that to me?