There are two different collections of music on this page.
The first is a completely reinvisioned soundtrack for UFO Defense. These mp3 files were composed and produced by Ziocody. He cut some pretty decent music from countless sources including several xcom games' soundtracks, deep drones, dark ambient music, and anything that resembles the sci-fi suspense mood set by this xcom. Now for the drawbacks: it only works in windows by muting the SW Synth under volume control to get rid of the game's music. You must run the music through an audio player so you can easily change the track as you change between Geoscape and Battlescape (to me it's easy as a quick alt+tab or windows key). If you have multimedia keys on your keyboard, you could make good use of them for this.
This music is taken from the Playstation version of X-COM: UFO Defense. Right-click and Save As to download them or click to play.