In MGS4, Snake has access to an iPod and has various methods of acquiring music to play on it. You can find some of the music just by playing through the game as some NPCs just give them to you. However, there are some that are hidden.

1. In Act 4 during the mission briefing, you can take control of the Mk. III (press SQUARE). Head upstairs to find an iPod song and a Battery.
2. In Act 4, after power is restored to the Nuclear Storage Building, head back the way you came, to a locked room in the Tank Hangar. It will now be unlocked. You can enter to find a hidden iPod tune, and a box of Suppressors.
3. During the mission briefing in Act 3, at a certain point you can take manual control of the Mk. II (Press SQUARE). Upstairs you'll find a Battery and an iPod tune.
4. During the Act 2 mission briefing, take control of the Mk. II, and upstairs you can find Noodles, a box of Ammunition and an iPod tune.

Some of the music is meant to reduce Snakes psyche gauge, while others can be used offensively or as heals. For example one iPod tune called Rock Me allows Snake to increase the effectiveness of healing items. Another tune called Policenaughts Ending will make soldiers fall asleep at your touch.