You can unlock more difficulty modes for subsequent playthroughs of the game. The more you complete the game, the harder the difficulties available will become.

To unlock New Game+, complete the game once through.

To unlock a Sound Test, which allows you to listen to all of the game's audio, complete the game once through.

To unlock Buren Wanzer for the OCU story line, beat the OCU story line.

To unlock TCK Assault Wanzer for the USN story line, beat the USN story line.

Complete the game and then select the "Next New Game" option to start a new game with all previously earned experience, skills, parts, and money. The "Sound Test" option will also become available. Additionally, depending on the number of the times the game has been completed and saved, the enemy HP is multiplied by the following amounts:

DIFFICULTY - ENEMY HP
x0.8 and x1.2 difficulties - One time
x0.5 and x1.5 difficulties - Two times
x2.0 difficulty - Three times
x5.0 difficulty - Four times
x10.0 difficulty - Five times