Rick, so you're not adjusting the volume (per your OP). You're adjusting speed, right?
The P4 decoders have a built in dynamic (chuff volume) adjustment. If you have momentum and turn the speed up you should get a loud chuff response and when you turn down the throttle even a little, the chuff mostly disappears, as if you closed the throttle, as it should. Are you sure the sound goes away when you turn UP the throttle? Perhaps there's a programming problem?
I have one of the BLI P4 Pacifics (and other that the ugly solid drivers) it behaves and sounds the way it should...
Otto K.