This is found on the multiplayer map High Ground, it can be done in forge and custom game( not sure about Matchmaking).
What you will want to do is go left of the huge missile launcher at the back of the map to the computer room. You will find a computer with a radio below, It will eventually stop making static and morse code and play random Halo songs throughout the series.
It may take some time for the music to play, it took me about 15 min of just standing my the radio, but if you are lucky it might play sooner. Good Luck :)
Edit: Thanks to PRlSTlNE for confirming that this is also obtainable in online MP.