To change a sound, you would need to get the "depbo" program from the ed depot and un-pbo your sounds.pbo (in the res folder). Then find which sound in there makes the "ding" sound, and replace it with a new sound (keeping the same name and file format). Then get the "make pbo" program, and make the folder back into a pbo. Make sure to backup your old sounds.pbo first.
The same sound is used by the hints, I believe. If I remember correctly, the old dynamic sound range replacement changed the hint sound to something quieter. You may wanna try using that, if you don't want to do it on your own. I think it is in the "tutorials" section of the ed depot; but do a search.