When I do long flights, I usually like to turn off the sound in the sim when I get to cruise (by pressing "Q") so that I can check on my flight and not have the noise of the engines fill my headphones suddenly when I switch back into the sim. I have noticed that when I do this for a long time (usually on flights of 4+ hours), the cockpit noises are gone when I turn the sound back on. That is, in the cockpit, I can still hear the engines, but can't hear any of the noises such as the flap handle moving, the autopilot disconnect alarm, callouts etc. In the external views, all the noises work just fine.
I have had this issue with both the 777 (in FSX:SE) and the 747 (in P3D v3). I have also checked all of the sound settings in the sim and in the FMC to ensure that all playback devices are setup correctly. I've never had the issue when I've kept the sound on for the whole flight.
Does anyone know of a fix for this slight issue, or why the issue is happening at all?