On my home 2 rail layout I have QSI Titan decoders with very close to prototype accurate horns, whistles, bells and chuffing, brakes, blowdown, etc. Each of the sounds is individually adjustable and I enjoy trying to get them balanced and then enjoy trying to find volume levels that seem as realistic as possible to me. Always a compromise between up close and far away. But, lotsa fun.
On our 3 rail club modular layout running my 3 rail engines I let 'r rip!! I join in the fray and crank 'em up all the way to compete with all the other engine sounds on 4 tracks and lots of accessories sounds too. And the general din at a train show. Man, with 4 trains running on that K-Line hollow tubular shadow rail and the carnival scene blaring we stir up ruckus. Kids of all ages love it!!!
3 rail --lots of smoke, too. Every now and then someone gripes about the smoke but we play like we don't hear them. Too much noise!!
I recently went to a layout tour in Ft Worth. Visited two large O Scale 2 Rail layouts. 40 X 50 or thereabouts. Hand laid steel rail. Heavy brass engines and brass cars rolling on that steel rail. Lights. No sound systems. But, those heavy engines and cars on that steel rail sounded -- well -- very realistic and satisfying. After all when we're at trackside railfanning a long consist we spend most of the time just listening to steel wheels on steel rail.
It's all good!!!