Unlockable Music Tracks in Sound Test
These tracks are obtainable via Standard Marathon Mode.

Ancient Tetris (Tetris GB - Music A) - Get to Level 19
Balloon Tetris (Touch Mode Theme) - Get to Level 14
Basement Tetris (SMB 1-2) - Get to Level 3
Bowser Battle (SMB 1-4) - Get to Level 10
CongraTetris - Finish Marathon (Get over 200 lines)
Mario Tetris 3 (SMB 3 Overworld) - Get to Level 4
Rushed Tetris - Get to Level 16
Tetris Climber (Ice Climber Theme) - Get to Level 15
Tetris DK (Push Mode Theme) - Get to Level 13

Endless Mode in Standard Mode
To unlock this, you have to finish Standard Mode. (Get over 200 lines) This will be accessible as a switch when picking the Level in Standard Marathon Mode.