This is an Easter Egg for the map High Ground that appeared in the Halo 3 Beta.
How to find it
To produce the egg, make a party of four in matchmaking and enter the 2v2 playlist, or produce a custom game. Make sure that none of the players have the intent of actually killing each other, so you won't have the annoyance of enemies shooting you while reproducing the egg. Keep vetoing maps until you find a game in High Ground. In the Base, there is a room with two large computer systems that can be destroyed. Next to the larger one (near the door to the bridge to the pipe room) there is a small green box. Upon closer examination with a sniper scope, you can see that it is a radio. Now, here is where the luck part of the egg comes in. Take out an Assault Rifle and shoot the small group of 4 silver circles on the right side of the green colored screen. If shot just right, the radio will stop hissing static and play what sounded like the song from Halo: Combat Evolved, Under The Cover Of Night. Otherwise, it will start playing something much more likely to happen, a message in morse code. The message says 'Frog Blast The Vent Core!' This is a phrase in which explosive human simulacrums would say in another Bungie game, Marathon.
The egg is easy to hear in Halo 3.
Halo 3
It is now known that this has been changed by Bungie. To hear the music, all you need to do is wait for a while in the computer room. The radio below the larger computer will eventually stop making static and morse code and will play random Halo songs (Halo 1, 2, or 3). You can tell it is about to play a song when it sounds like an FM radio just barely out of tune from a local station. Note, you can pick up the radio and place it in a different area in forge, and it will play music anyway.