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I fear you may find this answer disappointing. But it is 100+% fact-based.All that software on your computer can 'see', detect, etc., is any device plugged into it.
If the mikes are plugged into a mixer, and then the mixer is connected to the computer, the mikes cannot be 'seen' through the mixer. Their input via the mixer may be able to be processed, received, etc. But as devices, they are 'invisible' to the computer... which should be able to 'see'/detect the mixer, though.
Does that square with what you are seeing? Is the mixer detected by the computer, at least? it should be.
You have to have the mike(s) plugged directly into your computer, for them to register as 'devices' or 'microphones', etc. That's all there is to it.
Try to get Skype to see your mixer, perhaps? though since it is not a microphone device, Skype won't be able to select it as a microphone. Perhaps as an input device, anyway?