When you beat the game you unlock the BGM option. The BGM option can only be used in-game.
Easy money
Register five new survialists and set all their skill points to Chop. Put them in your party. Enter the Labyrinth and go north twice, then east. Go though the shortcut. Note: The shortcut must be activated form the other side before it is useable from this side. Next, go east, then north, and repeatedly press A until you reach your daily limit. After that, go back to town. Sell all the items and sleep until morning. Repeat this until you have as much money as needed.
Hexer Class
After you reached 4th Stratum to where you met Tlachtga in 16th floor and she will give you the Hex Bell. Go to the Explorer Guild and you will get the Hexer class.
Ronin Class
After you reached 3rd Stratum to where you met Ren in 11th floor and she will give you the Old Scroll. Go to the Explorer Guild and you will get the Ronin class.
Secret Ending
Press "LAARLAAR" while Atlus logo. It's a success if you can hear sound effects.