Every time I sign in iCloud I receive in iPad a notification to allow login. Thats ok!
What's not okay is that sound so loud. How do I make it elegantly silent?
The two-factor authentication alert follows the volume of
Ringer and Alerts. It can be adjusted via the hardware volume buttons, although you may have disabled that functionality.
If that is the case, head into Settings → Sounds and adjust the slider for
Ringer and Alerts. This will affect the volume of all notifications (including FaceTime call notifications).
If you want to completely silence those alerts, you can: