Set new audio device default for volume mixer in Windows 10

I use two audio devices:

• Default (audio) device "Realtec Digital Output (Optical)"
• Default communication device "Lautsprecher (Speaker)"

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My goal: I want to have the default audio volume on my default communication device, instead of the default device - at all times.

What I tried so far: Changing the volume mixer to my communication device explicitly works temporary, but as soon as I close the settings window, the default audio volume is reset again to the default audio device.

Step 1: Clicking on volume shows me the default device:

Step 2: Changing the volume mixer to my communication device manually:

Step 3: Clicking again on volume does not keep the change :(

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How can I make it permanent that windows uses my communication device for default volume control?

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