Music files are not distributed together with the
source code because of their size.
ASC can run without the music files being present, this message
is just to inform you about the existence of the music.

You can download the music files and install them manually to
${LOCALBASE}/share/games/asc/music

http://www.asc-hq.org/music/frontiers.ogg
http://www.asc-hq.org/music/time_to_strike.ogg
http://www.asc-hq.org/music/machine_wars.ogg