hmm, an all round square up like chr!s said is always good. Also, for the sound, try to keep your hand as close to the deck as possible without touching it, and theres no need to do it fast, it isnt the cardini.
Your hand covers a lot of the deck when doing the change, so you'd be amazed how slowly you can perform the move and still have the same effect. Try it in a mirror or something and see how slow you can do it before it's very obvious what you're doing. Also, I find a lot of the sound that happens sometimes when I do it comes not from the card moving but from the card hitting my cover hand. Experiment with different distances to see what helps you do the change as quiet as possible.
As for the squaring...instead of using an all around square up, I "clean it up" using a dribble or a riffle, which gives a reason for it not coming square to the pack and also because I do those things more naturally than the square up. You can try and work out a method for doing this easily or feel free to PM me and I can tell you mine.
I wouldn't worry about the sound to much. When I do it, I make it seem as if it changes by me flicking the card (I don't actually ever touch it with my right hand). So, they immediately assume that the sound it makes is my finger flicking the card. I know this from comments like "Can you do the thing where you flick a card and it changes?! HOT DAMN MAMA WOOO!!"