Right, I'm going to ignore the obvious Hollywood blockbusters and go for a film score by an established composer which was later used in another work.
So it's Henry V ( the 1944 Lawrence Olivier version ) by Sir William Walton or Scott of The Antartic (1947) by Ralph Vaughan Williams which lead to The Symphony Antartica.
I've born in mind that Mahlers 5th Symphony was used in 'Death In Venice', but I think we're talking about 'Original' Soundtracks here, not re-workings of established classics.
1."Burn" - The Cure (Robert Smith, Simon Gallup, Boris Williams, Perry Bamonte) – 6:39
2."Golgotha Tenement Blues" - Machines of Loving Grace (Scott Benzel, Mike Fisher, Stuart Kupers, Brad Kemp) – 4:01
3."Big Empty" - Stone Temple Pilots (Dean DeLeo, Scott Weiland) – 4:56
4."Dead Souls" - Nine Inch Nails (Joy Division) – 4:54
5."Darkness" - Rage Against the Machine (Rage Against the Machine) – 3:41
6."Color Me Once" - Violent Femmes (Gordon Gano, Brian Ritchie) – 4:09
7."Ghostrider" - Rollins Band (Suicide) (Martin Rev, Alan Vega) – 5:45
8."Milktoast" (also known as "Milquetoast") - Helmet (Page Nye Hamilton) – 3:59
9."The Badge" - Pantera (Poison Idea) – 3:54*
10."Slip Slide Melting" - For Love Not Lisa (For Love Not Lisa) – 5:47
11."After the Flesh" - My Life with the Thrill Kill Kult (Buzz McCoy, Groovie Mann) – 2:59
12."Snakedriver" - The Jesus and Mary Chain (William Reid, Jim Reid) – 3:41
13."Time Baby III" - Medicine (Jim Goodall, Brad Laner, Jim Putnam, Ed Ruscha, Beth Thompson) – 3:52
14."It Can't Rain All the Time" - Jane Siberry (Graeme Revell, Jane Siberry) – 5:34
* Uses samples from the 1976 film Taxi Driver