Why?

do you run it in window mode and/or the external version of ivap?

I have already put in a support ticket on this issue. I have two screens and have the CDU on the second screen and run in full screen mode. When I click on the CDU buttons the sound stops, on pushback there is no voice unless I re-click on the main screen. In addition, direct entry through the keyboard on the CDU does not work on the second screen. All of these issues are not present on other PMDG products or other addons that I have. Mark.

do you run it in window mode and/or the external version of ivap?

IVAP run in window mode.

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.

Same problem here.. But i agree, not a big deal.. Nothing to compare with the NGX experience..

As far as i can see, the sound is handled different with the NGX if you compare it to other aircrafts. I have some 5.1 Sound System and an USB Headset, if both is connected, i can hear the aircon, gpws, tcas... via my 5.1 system and engine sounds via my headset. So, once the window is not active, i can still hear one of them. Its not a bug. A good reason to do it like that would be performance (FSX does not have to load all the sounds). I am not good in explaining this i know, but thats what i think about..