I found an easy way to still get Active Display email notifications, and also having no notification sound played. I of couse still have volume on to hear ringing. What I did was to create a silence ogg sound file on my pc, of 2 secs, using a sound tool I have. I named this file Silence.ogg. Then on the Moto X hooked up to my PC, I just copied this file into the Notification folder. Then on your email settings, as notification sound, select "Silence".