Bart, my garratt is stored at the moment. I was hoping I could help without opening the wooden box so I checked the manual. The sound volume adjustment is on the tender. I believe this is the screw you are inquiring about.
Page 19 / Operation / Sound.
This model features a digital electronic
sound system. The volume control is located
on the rear of the coal bunker (Fig. 2). Use
a small screw driver to adjust the volume.
All sound features described below also
can be controlled directly with the LGB
I have some Lehman 1 amp power packs for testing on a short track and also an Aristo 10 amp.
It runs on the 1 amp but would be very under powered if it had to do any work.