What is the piece of music that plays during the trailer at this url:
http://www.movie-list.com/x/x-files-2.html

I am assuming that you want to identify the music that comes right
after Mulder says "Scully, run!"

Promos for movies often borrow snippets of other movies' soundtracks,
and that is the case here. The music comes from Craig Armstrong's
score for "William Shakespeare's Romeo + Juliet." Portions of "Escape
from Mantua" and "O Verona" are used in this X-Files movie trailer.

"Song Title: Escape from Mantua  

 Album: William Shakespeare's Romeo + Juliet: Music From 
 The Motion Picture, Volume 2  

 Used in: Some movie trailers for FTF [The X-Files: Fight the Future]

 Other: The full audio of the music contains a bit of 'O Verona' 
 (another music used in trailers) at the end."

Cached copy, The X-Files Music
http://web.archive.org/web/20041010053152/http://www.thexfilesmusic.com/mox/ftf.html

"Trailer Music
 The X-Files (1998)
 Romeo + Juliet (1996) - Craig Armstrong  
 Scream (1996) - Marco Beltrami  
 Original Trailer Music - Mark Snow"  

Soundtrack.net: Trailers: The X-Files
http://www.soundtrack.net/trailers/?cid=X&id=686

I hope this is precisely what you need to know. If anything is unclear
or incomplete, please request clarification; I'll be glad to offer
further assistance before you rate my answer.

Best regards,
pinkfreud

Thank you for the five stars and the tip!

~pinkfreud