Look on the 2nd-to-last page of the installation instructions that came with the card. It's got a wiring diagram for hooking the Aristo Revolution up to the MyLocoSound board. You take the appropriate function wire for whichever function button you want to trigger the horn (1 - 6), and hook that to the "H1" terminal on the MyLocoSound board. You mention the red and blue wires, which I'm assuming are two available wires from the accessory harness connected to the TE. Whichever button either of those two wires correspond to will trigger the horn (just make sure they're programmed to "momentary" on the TE controller.) The black wire from the accessory harness (according to the diagram) gets connected to ground. (You may not have to physically connect that wire to ground. I've done some installations where you do, others where you don't. It's weird. I did not test my MyLocoSound board with the Revolution, but I've got it hooked up to an RCS per their diagram, and also with a manual push-button to trigger the horn. Both installations work very well, so I have no reason to think the Revolution installation would be any different.
You'll want to make sure the jumpers are removed from LK1 and LK2 pins on the board, which is needed for controlling the board from R/C controls such as the Revolution. (Page 1 of the instructions shows where these are located.)
A few other things... I'm not sure how well the bell voltage input plays with the PWM signal coming from the TE, so you may or may not have the low-voltage control over the bell that you would like. Also, if you're using the bell, you're limited to a single-tone horn, which is actually two tones played one after the other--not terribly "American." IMO, it's better to just turn the bell off (it doesn't sound all that great to my ears) and run with the dual-tone horn instead. That's at least how I have mine set up.
Todd, in terms of sound quality, it's pretty good. It's not going to fool anyone up close and personal, but from 10' away through a "typical" speaker, it sounds very plausible. At that listening distance, you lose all of the "electronic" nuances of the synthesized sound, and are left with something that sounds very much like a generic internal-combustion engine. I've got mine installed in my little B'mann Davenport, playing through a 1.5" speaker. It's great background noise when I just want to see something simple running around the railroad while I'm doing yardwork.
Later,
K