To clarify bxaimc's question, BRSTM is a streamed format (similar to WAV, or CDA), and a BRSAR is an archive that stores various files, usually sound effects, or sample data. A BRSAR is NOT a streamed format, and is not a BRSTM. On the opposite end, a BRSTM is NOT and archive. Really, you shouldn't be associating them in the same sentence the way you are. Though, sometimes a BRSAR will contain BRSTM.
So, as near as I am able, the answer to your questions is "Theoretically you can edit a BRSAR, but no tool exists to do so." As for the BRSTM (a completely different question), I would say "Sort of. You can output the BRSTM to a WAV, edit that, and then re-encode it to a BRSTM. Personally, I wouldn't recommend this because there is likely going to be some loss in the audio quality."
oops, I meant to put brsar, not brstm. I know what each is, I just accidentally typed the wrong one because I deal with brstm files a whole lot more than I do brsar
I don't think we're quite at the point where brsar is understood well enough to rewrite it. It should be doable, it would just require some more work hacking at the format and probably a good deal of testing to make sure that the rebuilt brsar still works. It was doable with brstm, I expect it could be done again, even though brsar is a bit more complex it is about the same level of organization (and shares a lot in common structurally). But at the moment I don't think there are any tools to do it.
Has there been any update upon this? Has anyone tried to create a converter? I'm also interested to edit Brawl's smashbros_sound.brsar
It's important to the Smash community that this be doable, I personally would be willing to help in anyway, if needed. As you said "probably a good deal of testing"
I'd just like to know if anything could be done, or if it already has.
With the development of tools that can edit brawl character's movesets and a model/animation editor being released in the next month or so, people are starting to want to add, or at the least replace, audio clips.