I'm having the same trouble. I haven't used MCE before so I'm trying out the demo. I run XPlane 11 and Windows 10. Everything installed ok, and I have confirmed that the Sound In and Sound Out devices are all correct (both through Windows and the About tab).
Xplane and MCE load fine, everything seems ok, I'm using the FJS Q400 (latest version). First of all, when it loads it doesn't seem to go into Flight Mode. Forcing it via the context menu doesnt work, although going to the Quit XPlane screen does get it into Flight Mode.
Once in Flight Mode, I can see that my voice commands are recognised - they appear on the top left of the screen, but nothing happens. Looking through the options, I can Test the Voice for the Purser and The Mechanic, but not for the CO-Pilot. The most I've managed is to get the Flight Attendant to reply, so it does seem to work on some level, but there seems to be no functionality for the co-pilot at all.
I have noticed that, when MCE is running, there does seem to be a background hum that wasn't there before.
Any ideas? My trial runs out tomorrow evening and I haven't yet had a chance to try it out.
Cheers..