Aww. Thanks, Kira. Right back at'cha, baby.
Dusty, I'm going to be lousy explaining it, but basically, since it was a long-established command but only used infrequently as part of silly tricks, I made it more of a...uh...OB thing, I guess???? Just kind of made it more serious (like leave it, come), isolating it as its own "thing" in small, unexpected "drill" sessions, and marking the behavior (which he thought was super-cool since he already knew the command!). Also had him use the voice for anything he wanted...a Kong, dinner, etc.
The next step was when he used that non-stop, penetrating monster bark (as opposed to just a couple barks, or him being over-threshold), he was told "indoor voice." At this point, I was still marking the behavior, looking to re-condition the long-practiced response of maniacal barking.
Once I thought he was pretty solid, I felt confident (ha-ha) that I could invite him to look out the window
with me if someone was walking a dog, especially with little kids. He's extremely dog/animal/kid aggressive; the micro-second he'd lay eyes on a dog I'd calmly say indoor voice--he would, I'd mark. It got to the point where he'd see the trigger(s) then look for a treat instead of reacting. (In that way, he should've used the voice, I guess, instead of looking for a treat...unless he connected the dots...but he's so very aggressive, that's something I gladly let slide.)
Sometimes his indoor voice is so faint, you only know he's using it by seeing his "giddies" ripple or a movement in his throat. Other times, it's a succession of guttural mumbles. Cracks me up.
All this said, he still lets out those SINGLE barks that take ten years off your life.

They come from nowhere! 'Know the ones I'm talking about? I had a full cup of coffee the other day when he ripped off one of those...Mama wasn't happy.