I had this aggravating problem where turning on or off monitors suddenly caused the sound to dim.
I’m running Windows 7, but this seemed to be the solution to my problem:
In short, windows was lowering the volume because it thought that communication was taking place. If I had to take a guess, it’s because Windows 8 aims to be an O/S on all devices to the point that features commonly found on phones are being ported to the desktop. With annoying effects.