After the upgrade to 8.0, the Google+ app notifications sound change to the default notifications sound. I went through the settings to change the sound back the way it was but that setting is completely missing. The Ringtone option is not there anymore.
Is there a different way to change the notifications sound for this Google+ app?
Settings > Apps & notifications > App info > Google+ > App notifications
From here, you can enable/disable specific notifications, change their importance to a level with or without sound, and/or set the sound you want to use for notifications.
Here's what I was able to find for my Android 8 phone (no one told me this, it wasn't documented anywhere that I could find, I just kept searching). While in Gmail, go to Settings. Tap your Gmail account name (not your General Settings, although this will work from there also). Tap Manage Notifications. Tap Mail (make a sound) EVEN if the toggle switch is on indicating you have a sound set. Then tap Sound. Unlike in earlier versions of Android, you can not select a different sound for different "labels", just one sound for ANY label. I have the exact same version of Gmail on my Android 4.4.2 tablet and I can set different sounds for different labels by going to Settings, My Gmail account name and Manage Labels ? Seems strange to me that the same version of Gmail works differently on different versions of Android........but......
Hope this works for you also.