Thank you! This worked and also explained why I wasn't getting notifications at times. My issues started just prior to the Oreo update, and became worse after. But checking Accessibility/ Notification reminder (1 minute interval) fixed it today.
After trying all the possible fixes everywhere and making sure all the toggles and checkmarks were checked, I never even thought to look in the least place I would have looked.
Settings / Accessibility / Notification reminder
I may have forgotten about this section in my phone and at one point messed with it some time ago and possibly even turned it off as I was exploring the phone. Well I noticed it was off and no apps were allowed to remind me of notifications. I checked it on and I left it at 1 minute. I then waited for the list of apps to populate and began to check on all the apps I wanted to be reminded from. After I went back into the stock messaging app. I then sent myself a text message ( if you are on the messaging app it will not make a sound ) and quickly backed out of the application. To my surprise, I was immediately notified (with the messaging sound I had picked prior in the notification sounds) of the message I had sent myself.
Try this..... this may be a fix for you. I am on Android 7.0 on the S7. This should work for S7 Edge and possibly others. If not there may be something wrong with the phone.
GMAIL, OUTLOOK, SNAPCHAT, etc are all making notification sounds now. Unknowingly turning this off in this section had dissabled message notifications all together.