The above song from Mass Effect 1 only plays twice and only for a few seconds:
1. When Ashley sees a man get impaled on a Husk spike.
2. In the five second cutscene on Feros that shows a Geth Destroyer walking down a hall.
What rare and/or unused music in a video game is your favorite?
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The Prologue theme from Symphony of the Night. It only plays in one room, and Dance of Illusions takes over for the boss fight, meaning you're likely to hear only about 10 - 20 seconds of it. Yet it's one of the best songs on the soundtrack.