The 'Horde' themes can be found within /sound/music/zombat (here are 'snare_horde_01_01a' and 'b.wav', as well as 'gatesofhell.wav' for when a horde has been defeated). The 'Germ' musical cues can be found in the /sound/music/mob folder. 'Germ' plays when a horde attack is incoming, and they have many variations, such as on different instruments pertaining to the theme/setting of the campaign, or a pattern of going up in pitch or down in pitch.
And to be different from other people, I made my arrangements of the Horde themes unique by also including each campaign's 'Germ' tones as intros and outros. Since I'm not a video game engine, this music was done the second way. And they're arrangements by very nature of how the sounds are stored the game files: They're all in pieces that have to be stitched together either by the Source engine in real time during a game (by the 'AI Music Director'), or by a sound editor using software like Audacity. Yes, I know that many other people have created videos on YouTube sharing their arrangements of the Horde themes in Left 4 Dead 2.
