Many build their bench work and layouts in such a way as to deaden or lessen the sound. I can understand that if it's necessary so as not to disturb others, such as a hobbyist who lives in an apartment building or other multiple unit dwelling.
My preference is not to minimize, but instead maximize, the sounds and loudness of the trains I run with one exception.
In my experience, real trains, especially locomotives, are breathtakingly loud. I like that aspect of realism for all my O Gauge trains.
The only exception: I run quiet trains when my wife is asleep. Unlike me, she sleeps very well and has never had insomnia.
I often wake up in the middle of the night. When I do, I often like to go downstairs and run quiet trains, typically a Williams diesel or early Pre-Protosounds MTH engines, so as not to disturb my wife's sleep. After maybe 15 to 30 minutes of quiet train running, I usually return to bed and sleep for a couple of more hours.
How about you? Do you refer to run quiet or loud trains and why?
Also, what do, or did, you do, if anything, to increase or decrease the sound?