Siege of Madrigal

The Siege of Madrigal is a song that Bungie used in one of their previous games, Myth. One can hear this song when standing in a certain spot in the games Halo:Combat Evolved and Halo 3. This song is also is also featured after the last track on the Halo Soundtrack CD.

Halo:Combat Evolved
In the level Assault on the Control Room, there are a couple places where you can stand and hear the song. One can be found on the pyramid structure at the end of the level. There are three brackets on the large tower above the control room entrance. You will need to land on the second one with a Banshee to do this. Then once you have landed, face the cliff, go to the right side of the bracket and wait a few seconds. If you can't hear it, walk around a bit to find the right place.

Halo 2
On the Multiplayer Map Ivory Tower, stand by the metal door behind the elevator and wait. Attacking the door may also help, but be aware that the sound is very faint and hard to hear. Many have disputed the claim that the song does not appear on Ivory Tower; however, anyone with access to Halo 2's map files will find the song among one of the sound files for Ivory Tower.

Halo 3
In the final stretch of the The Covenant, after defeating the Scarabs, ignore the waypoint and fly toward the structure on the left (if looked at directly) that encompasses the building Truth is in. Carefully land the Hornet there (the return is possible without it, just much harder given the fall damage) and run along this ledge. Eventually, it will turn into a 45 degree slope; at the top, the song can be heard.

Warning: If you land on the building where Truth is, there is a possibility of activating the Revelation chapter prior to entering the building (The word will appear in the corner of the screen but nothing will happen). Should that happen (though to hear the song, you needn't go near the building itself) you will be able to continue the level normally until the point where the Flood forms ask to protect you and no further. The game will be stuck in the 'chapter title' mode with black bars across the screen and will not return control to you.

Trivia

 * There is a distinct resemblance between the Halo:Combat Evolved and Halo 3. Both require you to land on a precariously perched platform on a Forerunner building. It is unknown whether this is intentional, or purely coincidental.