Thanks for taking time to reply and look in to. As long as I know that almost any mouse click on the main panel gets my checklist sound back, it is not the end of the world. As I am not an FS2 Crew user, I rely on the built-in checklist on the airbus collection.
To address some inquiry from above: I do not use an additional sound cards. Using Nvidia Geforce GTX1080 graphics card. Solid state 1 TB hard drive, water cooled. Core I7 8700K 3.7 ghz. 16 gig ram. 8 gig video memory. Windows 10.
Again, when I click "shift 2" to pop-up main MCDU, then click "undock", it automatically moves that MCDU to my second monitor. Whether I leave it there or drag it back to my main monitor, does not change the outcome. The pilots voices in the airbus default checklist system, are muted. If you are in the know, any mouse click to the panel area, brings that sound feature back. Also, if you want to reverse the 'Undocking" of that MCDU, it can be tricky, as when you right click the top bar of MCDU, the "Dock" command is barely visible. If you are curious, if you were to click Shift 4 to bring of the features portion of the MCDU, the same sound muting problem arises.