iPhone new mail sound notifications not working

My phone does not play a sound to notify of incoming Exchange emails when using the native mail app.

I am using the latest software (iOS 15.5) and have all sounds / notification permissions turned on; including the custom notifications for Exchange. My email is set to push and my phone is not set to slient!

All other notifications (incoming calls, messages, WhatsApp's, Outlook emails via the Outlook app etc) all work fine...

Any ideas?