Have you tried changing the sound itself under Settings > Sounds. Also check under Settings > Notifications > Mail. Individual notification sounds can be set here for each account, and they override the general Sounds settings.
If none of that helps I'm not sure what is happening; but a couple questions that might narrow things down.
Is the Change with Buttons switch on or off?
When you tap the sound in the Sounds settings, is it louder, or the same as what you normally hear?
When you test the other sound settings, are they louder, quieter, or the same?
If you test the sound you usually use with Mail under one of the other notifications, how does it sound?