Hey there wllmst,
Thanks for reaching out to the Apple Support Communities.
Just to confirm, are you not hearing a notification sound for all conversations, or just particular conversations? If having issues with specific conversations, check out the steps below to ensure that the conversation was not muted: Change message notifications on iPhone - Apple Support
Mute notifications for a conversation
1. In the Messages list, touch and hold a conversation.
2. Tap Hide Alerts.
Another way to check if notifications may be muted would be to open the Notification Center by swiping down from the top of your iPhone screen, swipe left only slightly on the message notification to display Options or Delete. From there tap Options, and you should see an option to unmute if the notifications are muted.
Keep us updated on the results, and we'll continue from there.
Take care!