smashbros_sound.brsar doesn't work with Gecko. It only works with Sawnd files.
(Don't Worry You're Not The First Person To Have This Problem)
Make sure you have the right code
I think yours have the .bnk file soundreplacement bank engine code (or something like that)
Here's The Code for the sawnd files.
This isn't the same as the bnk code. This is longer
If that still doesn't work then try my tutorial.
http://forums.kc-mm.com/index.php?topic=65503.0