You are right, it's a copyright issue, some more info can be found in:
blobandconquer (1.11-dfsg+20-1) unstable; urgency=low
Music, sounds and textures are non-free.
Earlier versions of blobandconquer were released in Debian main with music,
sound and textures, since it was though that they were in the public domain or
otherwise freely licensed. However, it was brought to the attention of
upstream's main author that this was not the case for many files. The sound,
music and textures files have been downloaded from various free online
repositories, such as (but not limitted to) modarchive.com. These repositories
allow free use and redistribution of the files, but not necessarily
modification or commercial use. Therefore, the music, sounds and textures in
Blob and Conquer are not DFSG compliant, and cannot be distributed in Debian's
The game should still be playable, and in the future replacements will be found
for the removed game assets.
-- Guus Sliepen <email@example.com> Sat, 21 May 2011 14:38:59 +0200