Issue is with Rules in iCloud+
I do not have a fix for this, but can tell you (@Apple) that I can reproduce the problem 100% of the time on my devices. In a nutshell, applying rules in iCloud+ mail inhibits devices from playing sounds when mail is received.
Here are the parameters -
Environments - iPhone IOS 17.2.1 // iPad IOS 17.2 // MacBook Pro M2 Max OSx 14.2.1
Software - iPhone and iPad both have MS Outlook, Gmail, and Apple Mail set up. Microsoft Outlook and Gmail clients DO NOT have this issue and behave perfectly as expected.
Notifications are all set up correctly - NONE OF THE "FIXES" STATED IN THIS THREAD APPLY (even if user claims to have talked to support, or Apple rep, or whatever)
iCloud+ - Here I have 3 custom domains set up, each with a single associated email address (ie. userA@domainONE.com, userB@domainTWO.com, and userC@domainTHREE.com. There is also an iCloud+ user set; let's call this one mainUSER@icloud.com.
iCloud+ mail (NOT the Apple mail on device or laptop; this is the one you access through Safari / browser) -
--> 3 Folders set up (done by clicking the '+' to the right of 'Folders' in the iCloud web interface. There is a folder for each email address - ie. a folder for domainONEfolder, domainTWOfolder, and domainTHREEfolder.
--> Next 3 Rules are set up by clicking the 'Gear' icon (settings) then clicking "Settings", and "Rules" in the popup. Here the 3 rules set up are set up the exact same way to direct incoming emails to the proper Folder. So, emails sent to userA@domainONE.com automatically goes to the domainONEfolder folder; same is true for the other 2.
Now the issue ...
Emails sent to the icloud.com domain (eg. mainUSER@icloud.com) comes through fine on every device, complete WITH notification sound. Note that there is no rule that applies to this domain, so everything works as expected.
Emails sent to any of the other 3 domains - who have a rule associated that effectively moves the message into the appropriate folder - triggers a notification on the lock screen BUT HAS NO SOUND. So, an email sent to, say, userA@DomainONE.com is moved to the appropriate 'domainONEfolder' folder and then also shows up on the lock screen as a notification but generates NO sound.
Now, if the rule that applies to that same userA@domainONE.com (the one that moves the message to the proper folder) is removed, then taa-daa - when the email is received, a notification pops up on the lock screen WITH SOUND / chime.
I have confirmed this behavior with multiple devices, users, rules, and addresses. Each time, with the exact same outcome.
There you go @Apple - have tried my best to hold your hand and be as precise as I can with this in hopes you can get a fix turned around. Rules and chimes are critical to effectively using your platform to run my business communications.