How to change notification tone for all apps at once?

I created a new notification profile but I want to use an MP3 as the notification tone. I Googled how to set custom notification tones and it all that came up was how to set app specific notification tones. That's not what I want to do. I want to change the master notification tone within this profile.

I did it a few months ago, using a technique that could only be done with custom notification profiles (not any of the built-in ones) but now I want to create a new one and I can't find the webpage I used for instruction last time.

When I go into the settings for my newly made profile, I can't see any option to change the notification tone for all apps at once. How can it be done?