Well, the collectors edition of X-Com UFO and TFTD already have the music files in midi format. You could probably bung them into a mod tracker, put some apropriate samples in and convert it into an XM or IT mod if you like.
The playstation versions of X-Com UFO and TFTD have their music as music tracks that you can play on your cd player.
XCom Apocalypse's music file "music" in the root of the game cd is one gigantic lump of all the music in the game stored in, if I'm not mistaken, raw audio, just like all the sound effects. If you have a wave editor that can open the file, you could probably extract the individual music clips as wav files and then convert them into some other more manageable format like mp3 or whatnot.
I think Interceptor's music is in wave format, but in bits and pieces.
NKF, narrow minded fuddy duddy who refuses to let go of the past and will not accept anything newer than 1979.