Ok so i can almost 100% replicate this now.

Basically in the new windowed mode, if i am in the menu or inventory or something, and i mouse off the game onto one of my other monitors and do stuff on there, and then click back to the game then it almost always resets the sound to default.

Even if i change the sound settings so i still have sound, it will just ignore my settings and set it to default.
But it won't show changes to the settings.