Freeciv Music Set. For more information see README.SongOfDoom
To install the music set, open the modpack installer and enter this URL into box at the bottom of the window:
For freeciv-2.6: https://raw.githubusercontent.com/jwrober/SongOfDoom/master/SongOfDoom.modpack
For freeciv-3.0: https://raw.githubusercontent.com/jwrober/SongOfDoom/master/SongOfDoom.mpdl
and then click install modpack.