Instructions
Select "Extras" on the MGS4 title screen and select "Connect to the Network." After connecting, select "iPod Songs." A list of downloadable iPod songs appears on the screen. Highlight a song that you want to download and press the "X" button.
Open the "Extras" screen from the MGS4 title screen and select "Password." Enter any of the following passwords to put a new song on the iPod, all by composer Henry Gregson-Williams: "thomas" for the song "Desperate Chase," "george" for the song "Gekko," "theodore" for the song "Midnight Shadow" and "abraham" for the song "Mobs Alive." Use the password "mgo2play" for the song "Subsistence Action" by Kojima Productions Sounds. All of the passwords are case sensitive.
Play the game to find additional iPod music, which you receive by making certain achievements or exploring the game. A total of 34 iPod songs are placed throughout MGS4. Several websites, such as GameFAQs, IGN and Metal Gear Wiki have the locations of all of the iPod music in MGS4.