Same issue for me and was definitely not related to focus mode being turned on, Bluetooth being turned on, ringer being silenced, etc. I would know for sure as I’ve been working with mobile devices for almost 7 yrs now and 80% of my job is trouble shooting customer phones.
In settings I had messages set to appear on Lock Screen, Notification Center and Banners. I changed it to Lock Screen only and immediately message notification sound was back. I then turned Lock Screen and Notification Center on and sound is still working. It seems that having all 3, Lock Screen, Notification Center and Banners enabled all at the same time somehow disables the sound, at least that’s what I gathered. Hopefully this will be helpful!
I still can’t fathom why Apple decides to release “updates” that essentially downgrade the functionality of your phone. Then they have to release an another update, right after to fix the problems caused by the previous update just ridiculous, especially considering what us iPhone users pay for these devices! Why not smooth out the wrinkles before hastily releasing an update that’s going to cause iPhone users problems? Obviously updates are necessary, but try getting it right first!