Replace some of the music with the spy soundtrack from Splinter Cell 4 (Double Agent) by the same author of X-COM's OST, Michael McCann

The XCOM HQ gets the JBA infiltration tracks, 3 version of the same theme, each with increasing tension
    XCOM HQ Act 1 -> (Acoustic Guitar) NewYork_Infiltration_Part1 (4:24)
    XCOM HQ Act 2 -> (Acoustic Guitar) JBA_HQ_4                   (3:25)
    XCOM HQ Act 3 -> (Electric Guitar) NewYork_Infiltration_Part2 (3:45)

XCOM Mission Control uses the Splinter Cell main menu music

Exalt mission non-combat music is replaced by infiltration music from Splinter Cell
- remixed versions of the JBA HQ
- shanghai infiltration
- iceland infiltration

Backup the following files in your XEW/XComGame/CookedPCConsole folder:
HQSound_Act1Music_SF
HQSound_Act2Music_SF
HQSound_Act3Music_SF
HQSound_MCMusic_SF
SoundExaltSoundtrack01_SF
SoundExaltSoundtrack02_SF
SoundExaltSoundtrack03_SF

Then overwrite the files in XEW/XComGame/CookedPCConsole with the ones in the ZIP archive

The entire Splinter Cell 4 OST is avaliable for free at
http://www.behaviormusic.com/projects/splinter-cell-double-agent-soundtrack/