Setting Default Applications that are NOT standard or available in drop-down menu

In the elementary os AppCenter, there are two apps (i have installed) that can do that. They are: AppEditor and Menu editor. I recommend AppEditor over Menu Editor, because it lets you change app icons and the like. To give it the icon in the menu, turn on the show hidden entries button. then, in the search bar, search for the app.

Don't forget to make sure it has the proper category though (music player) or you will have to edit the desktop file.

As far as I know, if your app isn't shown in one of these drop-downs, it means that the app hasn't been properly registered in your system. You have to register your app in the system and create a [your-app-name].desktop file (those usually reside in /usr/share/applications or /local/share/applications directories).

A .desktop file is actually a text file, so you can open and edit it in any text editor. Among other things, a .desktop file should contain a Categories line, which states the category of the app (one or several), which helps the system to properly categorize the app. For instance, for a music player this line usually looks like this:

Categories=Audio;Music;Player;

If Guayadeque already has a .desktop file, please check if it contains the Categories line.

On how to register apps and create .desktop files in Ubuntu you can refer to this tutorial.