I looked up an old e-mail. Here is the solution from before:
Set the sound in the device settings/sounds/notifications.
Set TD/settings/Advanced/Notify on new mail to ON.
Set TD/Settings/Advanced/Email Alerts to No notification. (Turns off non-Rule notifications)
Set TD/Settings/Advanced/Manage Rules/[your rule]/Notification sound to ON and use system sounds.
Thanks to the folks at TD support for this information!