Track composed by Alexander Brandon
This track plays when you return to UNATCO HQ but not under the best of circumstances since by attempting to save your brother Paul Denton, you end up getting arrested by UNATCO and locked up in prison. The track has almost a East Asian feel to it which is probably no coincidence since your next mission takes you to Hong Kong. I feel it's definitely one of the better tracks in the game as it perfectly captures that sense of despair, betrayal yet also determination to break free from your shackles, no matter the cost.
The track's UMX file is called UNATCOReturn_Music.UMX which is the title I've employed here but it's also sometimes called "Enemy Within" or "Opponent Within" too.
I've extracted my favourite sections of the original UMX file which includes the normal ambient section followed by some very dance-able combat music at 1:51 and finishes off with the more reflective and contemplative conversation background music at 3:18.