Hey ...:::VILE:::..., I actually had managed to do this process successfully numerous times, but the sounds would become glitched each time, like you said you were having trouble with back when you were first working on this guide. However, I just found a fix for that problem, so that imported sounds do not become glitched/crackly.
The problem was caused by the importing of the newsound.spt file's data. Due to how badly I've wanted to edit .brsars, plus my analytical nature, I looked at everything I did in each step, and managed to pinpoint where the problem starts. Basically, the .spt files made by sndconv.exe are not formatted in the same way as the ones in the .brsar. So after I did a test of adding, removing, and rearranging various values in the .spt file to make it's formatting similar to the ones that you replace, I managed to finally get an imported sound to not glitch up. So it seems that like other game's .mdl0 files, .spt files have to be hexed in order to work.
I'll try and make a small guide on how to hex the spt files to work in Brawl's format ASAP. I also plan to get Brawl on my USB Loader(I have the ISO already) to record footage of the altered sound I did the test with being played in the game.
EDIT:
After a little more testing, I found that editing the spt file is actually not needed either.
Basically...
Everything highlighted in this pic is basically garbage and not to be put into the brsar.
Everything highlighted in black in this second pic is all that needs to be removed. Once removed, just replace it with everything not highlighted in the first pic.
That is literally all that's needed to make the sounds not glitch up in the brsar. I also loaded up an iso with the edited files to check if I did everything right, and the sounds I put in did not glitch up at all and sounded perfect.
Once I put in some more sounds, I'll start recording some video of the edited sounds in action.