System Sounds problem

How do I select and apply a Sound in an app? I have the Wise Reminder app which should display a Notification and make a Sound when a Reminder is due. It did for a while but has now stopped making a Sound, either as a Default or as a Sound chosen using the Custom option. The box for entering a Custom sound has a pre-installed . mp3 sound file which does not work, probably because that file is not listed in my System Sounds! The problem is, it can neither be deleted or replaced. Because of this what I thought would be quite straightforward has so far proved impossible. What is the procedure for selecting Sounds to be used by an app?

I have reinstalled it, twice with no effect. Clicking the ... to the right has no effect. Wise Reminder is a fairly small and simple utility among a large family of Wise apps, most notably Wise Care 365 and Wise Cleaner. I have been sent several fixes by their Help / Support team, none of which have worked --- which is why I am widening my search now. I thought there must be a standard basic way to select and use System Sounds, as if not what use are they? So that is what I am asking about here.

Thanks for your very informative reply. Before I had the chance to apply it I had to reinstall the app for the third time because it stopped working altogether and I got the same display as shown in the image you posted. So my earlier installations must have been corrupted in some way. This time the sound option works! If it happens again I now know where to look. Also, I realise selecting the option to repeat the Reminder every 5 mins after its first appearance should also help to stop it being overlooked. So far it seems that all the Reminders I set up before the latest installation are sans sound, so I will now see if I can redo them. Oops, it has just happened again.

While searching for a solution to the issue I checked all the Reminder apps in the Windows Store without finding one which had a sound option.