That's because MC captures the Media Keys to handle them depending on the configuration.
You can have MC ignore the volume keys by changing General > Advanced > Media Key Mode to "ignore volume".
You sir are awesome and correct!
I did what you said and the windows pop up volume now works!
Congratulations on your deep knowledge, because I would have never know about that setting but I do now, and so does anyone that reads this thread.
I have now set the MC32 volume to disabled for better sound quality.
I had it previously set to system so the keyboard buttons controlled it, but they now control the windows volume with pop up display.
So I get the best sound quality from MC32 with no internal volume and the windows volume pop up.
Many thanks again!