Hi Hombre-Lagosta,
glad to hear you'd be willing to contribute!
From my point of view UFO:AI has a lot of maps giving different moods, so it's not easy to simply say "make it that way". May be it's better if you have a look into the game if you didn't do already? In "skirmish" mode you can select from the maps to get a feeling.
I personally prefer tunes giving hope over those making sad. Anyway, amidst slaughtering aliens this may be absolutely right...
The songs should keep adrenaline level up.