Tetris DS Cheats

In-game reset:
Hold L + R + Select + Start while playing the game.

Standard Endless mode:
Get over 200 lines to After finishing Standard mode you will unlock the "Endless" option when selecting a level in Standard Marathon mode.

Music tracks:
After finishing the following tasks in Standard Marathon mode you will unlock the corresponding music track at the sound test menu.
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Ancient Tetris (Tetris GB Music A): Reach level 19
Balloon Tetris (Touch Mode Theme): Reach level 14
Basement Tetris (Super Mario Bros. 1-2): Reach level 3
Bowser Battle (Super Mario Bros. 1-4): Reach level 10
CongraTetris: Complete Standard Marathon mode
Mario Tetris 3 (Super Mario Bros. 3 Overworld): Reach level 4
Rushed Tetris: Reach level 16
Tetris Climber (Ice Climber Theme): Reach level 15
Tetris DK (Push Mode Theme): Reach level 13

 Tetris DS Official Website