Are you using custom sound files that you imported into the Notifications folder, or are you choosing sounds from the list that came with the 6P?
I've found that I can select any of the included sounds and they all work when I get a text message, but if I try to select one of my imported sounds, it defaults to the "Hangouts" sound included on the phone. It's strange because when you are in the Hangouts > Settings > SMS > Notification Sounds dialog, the custom sound does play correctly when you select it. But when you get an actual text, it defaults to the "Hangouts" tone. Leave a reply if you have the same experience.