Do you have version 6.4.42 updated 12/21/17? That seems to be the latest update. I'm still seeing the notification sound option via the three dots in the upper right-hand corner > customize > tones.
I was referencing what's on the Play Store listing and the app on one of my devices:
Apparently that page is out of date since I've seen a few posts this morning where the app was updated again a day or so ago to version 6.5.0. I'm not seeing that version on my device yet.
Which version do you have?
Joe Vallender wrote:
That instruction is for the Google Messages application not the Verizon Messages+ application.