If anyone else has been struggling with this as long as I have, I hope this is helpful. I've been trying to keep the "message sent" bubble and icon and lose the darn sound. I found a solution that works for me on my Mogul xv6800 here:
http://oldsap.blogspot.com/2006/06/s...tweak-ppc.html
I hope this works for someone else. It amounts to a pretty easy registry hack.