It's pretty simple once you know how to do it. First of all, take your smashbros_sound.brsar and place it in the folder you have Super Sawndz and its files in. I also suggest making a copy of your .brsar before you edit it.
Then open up Super Sawndz, you should now see a list of all the characters in the window to the right, I normally find the character i'm trying to edit and click on them so they are selected them but I don't think you need to do that. Then all you have to do from there is click the button that has a "..." on it above the insert button, find your .sawnd file, then click insert.
And boom, your done.
Oh I should also note that your .brsar goes in you pf/sound folder, then you have to put .sawnd file you inserted into a folder you have to create pf/sfx and name the .sawnd file the ID of the character its over, in your case you would name your file 21.sawnd