iOS text tone doesn't sound I just realized that my iPhone SE, iOS 12.3, isn't issuing the "ding" tone when texts are received. The Messages app shows a badge (small red circle) when a message arrives. The ring for a phone call works correctly. 

The text tone sounds correctly in Settings > Sounds > Text Tone, Do Not Disturb is turned off, and Settings > Notifications > Messages is set to "Allow." I've restarted (soft and hard), and the Mute switch on the side of the case is in the up (sound allowed) position. Selecting another text tone has no effect, the different tone is also not issued.

What else might I try?
It appears that if you have Messages in iCloud enabled on the iPhone, and you're also signed in to Messages on a Mac with the same Apple ID, and the Mac is logged in and awake, then the iPhone doesn't make an alert sound for new messages. Presumably the idea is that you're at the Mac and will see the message alert there.

I don't know of a way to configure this other than switching off Messages in iCloud on the phone.