Start the game NTW3 MP then once you see menu press Alt Tab. This will bring you out of NTW3 then go to bottom right hand corner. Click on speaker icon then click on mixer below that and you will see a list and you will notice that your steam volume is turned down to 0. Raise the bar up to a suitable level then click on NTW3 icon and the sound should return. You may have to repeat this process a few times.