Note: there's one more thing. paplay cannot play back mp3 files. Let's take it one more time so it does not get lost in the noise:
*** paplay can not play back mp3 files! ***
Mp3 is not in the list of supported file formats for paplay, and trying to play an mp3 back fails with "Failed to open audio file.". So even if you fix the helper to fix the xdg path stuff, you also need to convert /custom/usr/share/sounds/ubuntu/notifications/Aquaris.mp3 into a format (or use something else than paplay).
Note: there's one more thing. paplay cannot play back mp3 files. Let's take it one more time so it does not get lost in the noise:
*** paplay can not play back mp3 files! ***
Mp3 is not in the list of supported file formats for paplay, and trying to play an mp3 back fails with "Failed to open audio file.". So even if you fix the helper to fix the xdg path stuff, you also need to convert /custom/ usr/share/ sounds/ ubuntu/ notifications/ Aquaris. mp3 into a format (or use something else than paplay).