A simple little Doom-alike that was the first announced FPS for the GBA... but the fourth to be released.
Build Version#
At the title screen, hold down L or R, then press Start. A build number will appear at the main menu.
(Source: Cobra1 (GameFAQs))
Sound and Music Test#
Enter NRYRDDS_ (where _ is a blank space) in the password screen and press Start. You'll hear someone laugh, and cheat passwords will be enabled. Now enter CRSDR___ as your password and press Start again. Exit the password screen and go to the options mode. Turning the "Music" and "Sound FX" switches off and on again will play back a different song or sound effect whenever you do so.
(Source: XAnimal (GameFAQs))