I don't have an RT device, but I'm running Windows 8.1.
To get notification settings, you need to go to change PC settings>search and apps
Then go to notifications and make sure notification sounds are enabled
Then you'll need to select the Mail app to get notifications.
I have mine set to 'off', since I use Outlook, not Windows 8.1 Mail.
I'm running Windows 8.1 x64, but I don't think these settings should be different on RT, since the information about notifications is the same for desktop 8.1 and 8.1 RT on Microsoft's site.
How to manage notifications for Mail, Calendar, and People - Microsoft Windows Help