Personally I like Brawl's SFX a lot better. Actually they've gotten better with every smash.
I'd disagree to some extent. Electric sounds are
way too loud. Fire sounds in Melee were really good although Brawl did nicely by making them a bit louder. Brawl's common physical hit sounds completely lack the "Oomph" behind them that all of Melee's had from the "THUMP" on Falcon's Knee and Sheik's slap to the "THACK" from Mario's Fair to Ganondorf's Bair. Don't forget the replacement of Melee's high knockback flying sound with the annoying delayed wind noise. Of course, 64 didn't need it because it had the best audience clips.
Also, Brawl's common sword hit sounds (especially Marth's specific sword swing sounds) are absolute ****, no debate. Link's and Tink's swings are ok though. 64 personally did the best in this category.
Brawl did add a lot of other sounds to make each character feel more unique which is, for the most part, pretty good save for some things like how loud Fox's dash sound (amongst other added sounds he makes) is to dumb things like how Yoshi sounds like a rubber duck. And maybe it's just nostalgia getting to me but I liked the 64/Melee PSI sounds so much more.
Oh and finally, 64 is uncontested for most ridiculously violent sounding Ray Gun ever. I ****ing love it and I hate how weak sounding it is in both Melee and Brawl.