Halo music on High Ground
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 either the Halo: Combat Evolved song Under Cover of Night, or random Halo songs from throughout the trilogy.
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 that in Forge, mode you can pick up the radio and place it in a different location, it will still play music.