Is there a way to automatically decrease the sound on my sound equipped locomotives when they go into a tunnel?
No, but you can use F8 to mute the sound when you enter the tunnel and F8 again to turn it back on when you exit the tunnel.
Some clubs have "no chuffing" zones, where many trains are present at the same time.
Operators who do not know about F8 are quickly apprised by others when they enter a busy yard.
F8 Mute (CV135) may be changed to a non-zero value, such as CV135=16 to lower the sound volume. This allows operators to instantly lower the volume, while maintaining the sound capabilities of our models. Refer to the Digitrax Decoder Manual for additional information.