Siege of Madrigal

The Siege of Madrigal is a piece of music that Bungie used in one of their previous games, Myth: The Fallen Lords. Players can hear this music when standing in a certain spot in the games Halo: Combat Evolved (as well as Anniversary), Halo 2, Halo 3, Halo 3: ODST, and Halo: Reach. This music is also featured after the last track on the Halo: Original Soundtrack. It is one of very few Easter Eggs that can be found in every Bungie-produced game in the Halo series.

Halo: Combat Evolved
In the level Assault on the Control Room, there are a couple of places where the player can stand and hear this music.
 * 1) 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. The player will need to land on the second one with a Banshee to do this. Then once one has landed, face the cliff, go to the right side of the bracket. There should be a part of the bracket that points towards the cliff, then slopes down steeply. From the flat part of the bracket, move towards the steeply sloped portion slowly. Eventually, the music can be heard.
 * 2) Another is at the beginning of the level. If the player goes through the level Assault on the Control Room and get a Banshee, fly back, get out by the door to go backwards the way he/she came, and melee the Banshee through. Go all the way back to the spot where the cutscene was at the beginning of the level, and the big tube-like part of the level can be seen, that if one falls down without the Banshee one will die. Now fly down with the banshee to the first or second ledge, and if one stands in a certain spot one can hear it.

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 music does not appear on Ivory Tower; however, anyone with access to Halo 2's map files will find the music 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 left of the Citadel (the building that Truth is located 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 music can be heard. It plays almost immediately after one reach the slope. Unfortunately, it only plays a shorter version of the music, but it is easily recognized.

Warning: If the player lands on the building where the Prophet of 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 music, the player needn't go near the building itself) the player will be able to continue the level normally until the end of the cutscene. The game will be stuck in the 'chapter title' mode with black bars across the screen, and will not return control to the player.

Halo 3: ODST
The musicis found on the level Kikowani Station on all difficulties. It can be found in the third open area of the level, on the right side rooftop on top of a small bridge, which can be found by turning right when exiting the second tunnel. It can easily be spotted by looking for a large red sign with the words "ACCESS DENIED" printed on the left. Standing on the very middle of the area, while touching the wall causes "Siege of the Madrigal" to play as well, moving or standing anywhere else will not allow the music to play. On Legendary, when standing on top of the small roof above the door on top of the bridge, an animation of Martin O'Donnell dancing along with winged hearts similar to the Halo: Reach armor effect flying around him, as well as a dance beat will play. The winged hearts effect is used by Steve Cotton to test glitches, and someone seemed to have added the effect on the easter egg as a prank on Martin O'Donnell.

Halo: Reach
The Siege of Madrigal in Halo: Reach can be heard in Club Errera, a club that is located in New Alexandria.

The Siege of Madrigal can also be heard in Dr. Halsey's Lab, the secret room in the level The Package.

Halo: Combat Evolved Anniversary
The music appears just as it did in the original game, however there are two versions, "Classic" and "Remastered," depending on the music setting chosen by the player.

Halo: The Master Chief Collection
The music appears again, as it did in the original versions of Halo: Combat Evolved Anniversary, Halo 2, and Halo 3. Activating these music cues will unlock the Madrigal Debut, Siege of Ivory Tower, and Dirge of Madrigal achievements, respectively. It has been remastered into Spartan's Regret in the Halo 2 Anniversary Original Soundtrack.

Trivia

 * There is a distinct resemblance between the Halo: Combat Evolved and Halo 3 methods and areas to find the music. Both requires the player to land on a precariously perched platform on a Forerunner building. It is unknown whether this is intentional, or purely coincidental as the two games have several similarities.
 * In Halo 3, if you make it to the other ledge, there is another easter egg.
 * The finding of the Halo 3: ODST location was a result of a small contest held on the Bungie.net chapter known as "The Marty Army", where the first person that posted a video on their fileshare of themselves finding the music and completing the level would win a signed copy of the Halo 3: ODST Original Soundtrack.