As you probably don't know, De_Apehouse and Xmas_Apehouse_S actually has soundtrack, different based on which one you're playing! The music player in the Bomb Site room plays tunes when +use'd, which ties in with it's plot on Gamebanana (The Terrorists, hired mercenaries, must destroy the computer so that the FBI doesn't find all the pirated media on it.)

Some of the loops are hard to ID, but I went and got them all for you.
The format is: Artist - Title (Album, Year, Genre)

APEHOUSE:
RJD2 - Ghostwriter (Deadringer, 2002, Instrumental Hip-Hop)
Architecture in Helsinki - One Heavy February (Fingers Crossed, 2003, Indie Pop)
Shpongle - Schmaltz Herring (Nothing Lasts... But Nothing Is Lost, 2005, Psybient)
311 - Sweet (311, 1995, Rock)

-

CHRISTMAS APEHOUSE
311 - Blizza (Soundsystem B-Sides, 1999, ???)
Telefon Tel Aviv - Fahrenheit Fair Enough (Fahrenheit Fair Enough, 2001, IDM)
Coheed and Cambria - The Willing Well II: From Fear Through the Eyes of Madness (Good Apollo, I'm Burning Star IV, Volume One: From Fear Through the Eyes of Madness, 2005, Progressive Rock)
Bad Brains - I Luv I Jah (Bad Brains, 1982, Reggae)
Dream Theater - Scene Two: I. Overture 1928 (Metropolis Pt. 2: Scenes from a Memory)

These songs are certainly period accurate - the latest are from 2005, with the map releasing in 2006. (Reading that makes me feel old, even though I was just a little kid at the time.)

I hope you enjoyed this list! No credit is required, as sourcing some of these probably haunted me for just as long as it did you, if it did at all. I honestly thought that Sweet was just a stock loop at one point. It was worth it to track them down, however; some of these have become favorites of mine as well!

I picked up from an internet archive of Ape's old wordpress and the Nipper Maps website that HomicidalApe also made a map called de_abstractplane which also had music. Watch this space, I might add those tracks...