The problem is the cvar s3d_rolloff_factor.
Either bring down the console in the game and type s3d_rolloff_factor .005 or open in your basev directory in whatever game you wanna set the sound in the config.cfg file and change it to reflect that value...
Well it can be anything in tht neighborhood the lower you set the number the louder the sound becomes. The closer to one it the closer it is to the default.
And as a side bug (feature?) note. All rocket explosions sound at full volume no stereo as if they occured right on top of the player. its a bit strange sounding or was it this way with the regular doom?