There doesn't appear to be a way to get rid of the annoying notification sound when WiFi is available on the Incredible 4G (short of rooting, I guess). Can't find an option for "no sound" or "silent" among the choices for notifications. The wife was ignoring her incoming texts and emails b/c she heard the WiFi notification sound so often that she started to ignore all notifications. Did a search on many forums and reviews, and it seems that HTC and/or Verizon is cramming this WiFi stuff down our throats. The only way I found to alleviate the problem is to select the shortest, most inconspicuous sound for the default notification (Settings, Sound, Notification Sound, "Hint", Apply), and then select longer, more distinct sounds for email (Gmail, Menu, Settings, <my email address>, Ringtone & Vibrate, Ringtone, <select one>, Apply) and a totally different one for texts (Text app, Menu, Settings, Notifications, Notification Sound, <select one>, Apply).
To compound the issue, I have to select even different sounds on my Rezound, so that when we are in the same place, we know whose phone is sounding off. Maybe an app with a male and female voice, announcing an incoming text or email, would be better. :>)