Battlemaniacs

Battlemaniacs
Developer: Syrox Developments Publisher: Tec Toy Platform: Sega Master System Released in BR: November 1996 This game has a hidden level select.

A licensed port of Battletoads in Battlemaniacs with an interesting backstory which eventually led the game to be released only in Brazil (and in an unfinished state, at that).

Level Select

In the short period before the opening text appears, press Up, Down, Up, Down, Left, Right and listen for a sound. Alternately, set RAM value 757 to anything other than 00. Start the game afterward for the Level Select.

Note, there's a 94 frame (about 1.88 seconds) delay after resetting or booting the game before the code can be input, then the player only has 60 frames (1.2 seconds) to punch it in.

(Source: GameFAQs (Mezmorize99), original TCRF research)