I am having the same problem, when I extract the FMC DEU on my second monitor, then activating a key on the DEU with the mouse, the sound changes, It is the environment sound not the engines, when I go back to the VC on monitor Nr.1, just pressing the mouse button restores the sound volume back again.Then back on the DEU activating a button, the sound goes off again, it can be repeated all th time. Otherwise all the flights made up to now are all OK and I am glad to fly the NGX.