BK is right, but some of the thread has been devoted to how well the decoder will do this. It seems some sound decoders emulate what is happening with the loco well:trains: whereas others don't respond that quickly to what the locomotive is doing (you don't want the sound of a train at low speed if you have already accelerated halfway down the track, nor the squeal of brakes AFTER you've stopped)
Other points raised concern manual notching of the engine (the ability of the operator to ask the sound chip to output the engine at high revs .i.e. high tractive effort, even when it is at low speed, e.g the loco is trying to start moving with a heavy train behind, or conversely is at high speed but you want the sound to diminish as it is coasting down a grade)
More esoteric points concern the accuracy of the sounds produced for any particular loco, best listen to the gurus on here who have more finely tuned ears than me:disco:.