Do your custom notification tones keep reverting to default on Hangouts? This might be why.

[B. Diddy]

I've had occasional issues with Hangouts using too much battery, which would only resolve if I cleared cache/data from the app. But then I'd notice that afterwards, my custom notification tones wouldn't play, and it would go back to the default Hangouts tone, even if I selected the custom tone in the settings. Then some time afterwards, maybe several days later, the custom tones would reappear.

Then I realized that due to the way permissions are done in Marshmallow and above, when I was clearing the app data, I was also resetting the app's permissions. One of those permissions is to access my media/photo files. Since I reset that permission, Hangouts could no longer access my custom tone file. What made me realize this is when I took a photo using Hangouts to send to someone, it then prompted me to grant permission to access media/photo files, and then right after that, the custom tones kicked in again.

Bottom line: If your custom tones are no longer playing, go to Settings>Apps, select the app, then Permissions, then make sure it has access to your Media files.