I agree with lefteye. I totally avoid using public restrooms to pitch a loaf at all costs. Hell, I already turn into a kung-fu master just to tinkle... kick the door open, kick the door closed,...
dude... you're pooting into basically a porcelain tuba... so to muffle that, you'd need acoustic baffles on the inside of the porcelain god and perhaps multiple angles to diffuse the sound. And...