What I was thinking you could do if there is no better method is this
First of all make a folder and name it Music, put all your music inside it and then put that in Gamemode_Whatever
Next if you haven't got one already, make a server.cs file and in it put this code
for(%i = findFirstFile("Add-Ons/Gamemode_GamemodeNameHere/Music/*.ogg");%i!$="";%i=findNextFile("Add-Ons/Gamemode_GamemodeNameHere/Music/*.ogg"))
{
%filename = filename(%i);
filecopy(%i,"Add-Ons/Music/"@%filename@"");
}
Replace Gamemode_GamemodeNameHere with whatever you gamemode file is called.
That will copy all the music files from the Music folder inside your gamemode file and put them in Add-Ons/Music, then you could probably just add the music into Gamemode.txt. Remember to put ADDON GameMode_Whatever into your gamemode.txt as well so that the server.cs is executed.