I don't think there's a way to turn it off, soz. I know the sound you're talking about, doesn't bother me personally but it's only in the Object Editor, right? The only thing I can suggest is that you turn off your volume while working in the Object Editor.
I did try searching through the Preferences options under File, but even if you set all the Sound options to 0, you still get the ding. I'll keep an eye out for anything that may stop the 'dinging' sound, but it looks like you may have to grin and bear it or go with no sound at all if it annoys you that much, sry :/