Open Settings on your iPhone
Tap Notifications
Tap Announce Notifications
Turn off Announce Notifications
Or turn off Announce Messages with Siri
To stop it only on AirPods, turn off Headphones under Announce Notifications
Open Settings
Tap Siri & Search
Turn off Announce Notifications
Open Control Center
Press and hold the volume control
Tap Announce Notifications to disable it
If needed, open the AirPods settings in Bluetooth and check Siri-related options
Make sure your AirPods are connected before changing the setting