Saying that you've checked every setting you can think of, sounds like you've looked at both the device settings and the app settings, is this correct? Do you not want any sounds at all? Try setting the device settings to silent for notification sounds. If you are only interested in stopping email sounds and you've checked the both device and app settings, you may have an update bug. First try clearing the cache. This may also work if you are using the stock app. If you still have issues try clearing the app data. This may cause you to have to enter some information again. This may also work for the stock app. If you are using a third party app and neither of the other two clearing methods helped, try uninstalling the app and install it again. Of course after this you will have to enter all of your account information again. Sometimes updates get buggy and installing the app as a whole may solve your issue. If you are using the stock app of course you cannot uninstall it but you can uninstall the updates for it and then install them again. You won't loose your account information as with uninstalling the entire app as you would with a third party app. Be sure to restart the device between clears and between any uninstalling or installing. This will ensure that old data isn't still being used even though it technically has been removed. This happens sometimes with ghost files.
One thing I didn't think to ask or mention because I figured you would have noticed and possibly mentioned, but the notification is from the app you are using for email correct? Not from one that you are not using? If you don't disable apps that you aren't using in your app manager, or if they update and you aren't checking the settings in all of your apps installed for email you may be overlooking something. For example, most users by default when setting up their device enter their Google account during the setup process, and then go on to use a third party app. Then the Google app and/or the stock mail app captures that log in and begins accepting email. For most this causes dule notifications, or triple notifications if a third party app is thrown into the mix. It is safe to disable one or both of the email apps, in your app manager, that came pre-installed on the device if you are not using either for email.