There are many articles online explaining how to change the default notification sound, but I found it hard to find one which explains how to do it for individual / specific apps.
There are many articles which simply (and unhelpfully) say that the app should have its own settings, so for those that don’t, here’s how:
There are quite a lot of steps here, and one in particular is not obvious.
It’s easiest to demonstrate with screenshots.
I suggest you open the relevant app just prior to following this guide.
Open the main Android Settings Page by dragging down from above the top of the screen.
Tap “Apps & notifications”
Select the relevant app, probably from the recent apps list, but you might have to scroll through the list of all apps.
Make sure “Show notifications” is turned on, and tap the relevant category (there may be only one), this is not just a checkbox.
Make sure Show notifications is turned on here too…
…and that the behavior includes sound.
Select your sound, and you’re done.