Sadly, I do not have a "ERR Railsounds Dash-9 sound kit 200-106." on hand for testing.
I've done several steam installs and also the one diesel I have is the "ERR Diesel RailSounds FM series"
Oops, I misspoke. Honestly sorry, my testing was flawed.
Adjusting volume using the CAB2 volume buttons regardless of aux1 status adjusts prime mover engine sounds.
Bell and Horn are adjusted using the physical volume control on the board, which also then limits the max volume of prime mover.
Again, sorry, I thought it worked the other way.
This forum topic confirms that https://ogrforum.com/...d-control-using-cab2
On the older TMCC product, the CAB controls the background sounds, chuffing, prime mover, etc. The bell, dialog and whistle are only controlled via the physical volume knob on the locomotive.
In this case, with the ERR rail sounds, that's the volume pot on the board unless you modify and install an external pot.