You realize that in order to have new sound slots, you'd have to find a way to add those banks and references to the .brsar AND somehow make them have their own sound IDs right?
Sound IDs don't just come up from thin air, they must exist in the .brsar in some form.
The sound id is the index of the sound in the INFO entry list in the RSAR, yes. The sound id is called "InfoIndex" in the properties when you select a sound in Brawlbox, but it is not editable.
Unfortunately in order to add, remove or rename sounds, the SYMB binary tree needs to be regenerated. I never figured out how to do it but BRRES files use a similar tree that Kryal was able to write a complex (imo) generator for. If the binary tree ids are left at 0, the RSAR works on the menu but the game freezes when the match starts, I assume because the menu's audio accessors are coded differently than when actually playing.
Right now, the tree ids from the original RSAR are copied over to the rebuilt one.
All attempts at generating a new tree are commented out in here. I was working with Tempus but we couldn't get it generating properly.
https://code.google.com/p/brawltools2/source/browse/trunk/BrawlLib/Wii/Audio/RSARConverter.cs