Had the same issue for a while.  No sound on the Verizon Message+ app. No problem on other message apps, including the stock Samsung app. Carefully made sure that notications were activate in app settings, volume was turned up, etc.  Still, no luck.

Today I Finally figured it out. 

Take these steps to get Notification sound to work for the Verizon Message+ app:

1. Hold down the icon for the Message+ app.

2. Choose Disable from dropdown menu

3. Open Play Store and search: Verizon Messages

4. Choose Verizon Messages from list.

5. Choose Uninstall.  App will uninstall.

6. Choose Update.   App will redownload and update.

7. Go to Android settings -> Notifications -> Message+ and make sure that notifications are turned on (allowed) for Messge+ in the app list.

Notification sounds and vibrations should now work!

Customize the app, now that you downloaded the updated version:

1. Open Message+ app in the alphabetical list of apps by clicking the icon

2. Click the three lines in the top left corner, next to “Conversations” and choose Settings from drop down menu.

3. Look through the list of options and check off the features you prefer.

I like to check off: Enable Popup and Enable Popup Over Lock Screen.

This is an option which is unique to the Message+ app, and sets it apart from the other message apps, in my opinion.  The popup window, with the text content, appears - at the top level of your screen - so, regardless of what you had open, you won’t miss or overlook a received text. And, you can then reply from the popup window, without having to separately open the Message+ app.