FMike,
The volume is this:
"/ikalizer_LimiterLevel", which I have at 1, but default is 0.
I had it at 2 and the explosions were awesomely loud, draining out Vent and people. Now at 1 it does it only when lots of grenades are going off, so I would suggest turning Vent up well above normal if you want to have it at 1 or 2.