Unlockable: Boss on Parade mode
Complete the game on any difficulty.

Unlockable: Stage Select mode
Clear the Boss on Parade mode.

Unlockable: God difficulty
Finish level 10 on Hard mode. This should unlock the God difficulty setting.

Unlockable: Sound Test
You must get a high score in the Normal, 2-Minute, and 5-Minute modes with all 3 ships in the game.