After updating to iOS 15, a number of users have discovered that they are no longer hearing notification sounds for the Mail app. To fix this issue, you may need to adjust your Mail notification settings.
Back up to the Customize Notifications screen and repeat steps 3 through 5 for each email account you wish to configure, and then you should then start receiving notification sounds for incoming emails once again.
Related Roundups: iOS 15, iPadOS 15Related Forum: iOS 15