Email sound notification

I'm not sure what to suggest here. In Settings > Sounds & Haptics what have you set to your mail notification sound? In the same place, do you have Change with Buttons turned on or off? If you have it turned off, the tone sound will be fixed at the level of ringer and alert volume you see in the slider bar. If you have it turned on, it may not be loud enough to be heard. I have Change with Buttons OFF always.

One thing to add that your response is missing. With Mail, sound notification can be assigned per account if you do have more than one email account. You do that through Settings - Notification- Mail - [Account] and settings here will override what you have set in Settings- Sounds & Haptics - Mail.

Do you wear Apple Watch?

OK. Since you've spoken with Apple we have no clue what you've tried or not tried.

What have you tried?