So, turns out that the background singers are like, hardcoded into this song's instrumental. So instead of using the one from the show I had to use Mandopony's recreation. It's not a bad one, but the one right from the show has more kick to it. Still didn't turn out too bad, I think. Better than it could've been. Plus, a 15-track all-Bronymike chorus at the end is always nice.

Mando's instrumental: https://www.youtube.com/watch?v=UUOSulm9YRA
MP3: http://www.mediafire.com/play/3uh75ymdjq61b7c/Smile_(BronyMike_Cover).mp3