You know, if you're going to use Aliens music, I suggest also trying out the one called "Going after Newt"! It has a great start and then calms down but always has this tempo of urgency, perfect for a Spy mission on the Shivans, or a large assault and waiting for something to happen. I also got a whole list of movie music I was going to use in an now defunct campaign.
Basically anything from Aliens is good. Danny Elfman's Planet of the Apes "main Title" theme is excelent for a shivan onslaught mission, cause it has this alien raspy drum thing going which I love and then it goes all hectic, perfect for heat of battle. And if you want to go for that 80's cliché action hero B-sci-fi movie appeal to teens feel, I suggest Megadeth's "99 ways to die". Great song, but damn funny when used in a freespace mission.

Orff's "Mars" is pritty good, if not used

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