Unlock Music
Have MGS4 savegame on your stick at the end of the game to unlock all music tracks.
Unlock Ranks
Bear - Have a large amount of CQC attacks given.
Pigeon - Perfers non lethal attacks.
Rat - Falls for a lot of traps.
Sloth - Gets shot in the head a lot.
Flying Squirrel - Have a high percentage of rolling.
Jaws - Large Amount of Knife Kills.
Kerotan - Prefers Capture rules.
Night Owl - Use the ENVG a lot.
Bee - Uses lots of scans.
Crocodile - Very good kill death ratio.
Eagle - Large percents of headshots.
Snake - Perfers sneaking missions.
Turtle - Use the Cardboard box a lot.
Singing
During gameplay, in the text menu (on either all or team) type <SONG> for your character to hum/whitsle/sing a part the Metal Gear Online melody.