Open your messaging app and click the 3 dots at the top right of the screen. Then go to settings, notification, notification sound.
Weird. I'm on AT&T. I guess someone with Verizon will have to chime in.I'm on Verizon and do not get that on the 3 dots. I get 3 choices, scheduled messages, customize and hide nav bar
Maybe its under customize for u?under the message app