you can get a chuff sound into the loco that will use voltage to trigger the sound. It works pretty well, in fact there's a writeup on how to adjust the sound on this forum, you'll have to do a search for it.
BTW: I got the chuff working once I hooked up a reed switch and some micromagnets to an axle on the 4449's tender, which is where the LMS3-G is located. The purple wire goes to one side of the switch, the other gets 5V. For the loco on the HC2, there's a setting on the page after the button selection that let's you decide when to trigger the input. Very easy and the sound is awesome.