How do I turn off audio notifications for Google+ (mail and groups) on a Samsung Galaxy Note 3?
Start up Google+
Goto top left corner, to the overflow button (3 dots).
Tap on settings
Tap on account settings for which account you want to change notification settings.
Tap on notification.
Change sound file to an empty sound file. (To do that start voice recorder app and record a second of silence, then save and use).
This can be done on all devices with Google+.