Try this... Issue solved!
Hey any idea about how to get a message preview on the home and lock screen. My text messages show up as an icon but Viber and messenger only show at the top when unlock and go into the phone.
This is not a solution. All settings are as shown in the video and still no notification sounds. Unable to even preview the sounds in the default messaging app. Please investigate this further as it's unacceptable for a flagship phone to have this issue.
I had the same problem for a week. I reset my app preferences by going to settings>apps>3dots on top right>reset app preferences. Then I went back in and set up the apps I wanted notifications from. I started getting notifications from all apps, but there was no sound. I have the CM Security Master app on my phone, it was giving me notifications as well. I clicked on the notification, it took me inside the app. I then clicked on "NOTIFICATION CLEANER" then click on settings in top right corner. Turn On>Apps-Don't Disturb>ALL of my apps were checked. This is why I wasn't getting any notifications at all. I unchecked the apps I wanted to get notifications from and it has been working fine. Remind you If you have the CM Security Master App, this could be the problem. It was for me.
I solved my problem by presing volume button , then in the slider appered for ringtone clicked "V" icon - additional sliider appeared and Notification slider was on 0 position - I set it higher position - it works now!