You can also make a list of all the music files available so that you can actually select music from a list with your add-on. On your GUI you can make a GuiScrollCtrl, and inside that GuiScrollCtrl (as a child object), you can add a GuiTextListCtrl and name it something like "mpList", for example.
I think you can build a list of music files and add them to a list like this, correct me if I made a mistake -
function buildMusicTracks()
{
for(%m = findFirstFile("Add-Ons/Music/*.ogg"); %m !$= ""; %m = findNextFile("Add-Ons/Music/*.ogg"))
{
%musicFile = strReplace(fileName(%m), ".ogg", ""); //take out the OGG extension
%musicFinal = strReplace(%musicFile, "_", " "); //take out any underscores
mpList.addRow(1, %musicFinal);
}
}