SFX hacks are really easy with SuperSawndz and the RSBE code (replacement sound bank engine). I found out how to do it myself honestly.
But yeah, you can't open .sawnd files. They're meant to only be used and not edited... You make your own .sawnd file by using SuperSawndz.
Alright, first make sure you got your .brsar file in the same directory as SuperSawndz, and then start up SuperSawndz.
Announcer calls are in 4 different soundbanks:
menu
melee
menu
characall
Fortunately, they're all near each other on the list... For an easy way to find them, click on one of the names on the right window so that you can highlight them, and then just press "m" on your keyboard 'til it takes you to "menu".
Once you find "menu", click on the "+" buttons and then click on the 3rd from the top "+" button. Then you can click on all of the "audio" files and hear what they are. From here, go ahead and listen to the other announcer soundbanks, "melee", "menu", and "characall". (Yes, there is a second 'menu' soundbank)
Now to replace them. Highlight the audio file you want to replace, and click on the "..." button on the top left under 'Input File'.
Select the .wav you want to put in over the old one... and then press "Insert" and SuperSawndz will work it's magic. Repeat this for all of the .wavs you wanna put in.
OK, now I typed all that up before realizing you aren't using RSBE.
So you're just sticking with editing your .brsar right? I forgot why I prefer RSBE, but sticking with the .brsar's even easier. After you insert new SFX with SuperSawndz, I think it saves the changes to your .brsar automatically. So once you're done inserting the new sfx, just make sure you move the .brsar in your SuperSawndz folder onto where it needs to be in the SD card.