Did you ask permission to use Apocalypse music?
Because I wanted to use it too, but it is likely copyrighted.
As you should be aware OpenXcom doesn't include the original game graphics-music, you have to provide them yourself by owning the original game.
I'm a ignorant about this stuff but AFAIK otherwise it would be redistribution of copyrighted content (which requires permission).
Unless the music is re-arranged or somehow re-made, I think.
Original composer John Broomhall has a twitter account. Maybe asking him? Maybe he has old unreleased music for UFO/Apocalypse that he could donate to the cause?
