Turning off just email notification sound
The easiest way to achieve this probably is:


get yourself an "empty" mp3 (i.e. one without any sound).
on your device's SD card, create a folder named media/audio/notifications (if it's not already there).
copy that "empty MP3" into this folder
Now select this file as notification sound for new mail


So when the new mail notification sound is being played, you won't hear it anymore :)