In the first map, in the Options Menu/Language/Voices, select the "RAYMANIAN" language (don't validate it), keep L1 + R1 pressed and do the following sequence: L2, R2, L2, R2, L2, R2. A new menu will appear with the three multiplayer bonus maps.
Other cheats
In any other map, go to the Sound Menu, select the "MUTE" option (again don't validate it), keep L1 + R1 pressed and do the same sequence: L2, R2, L2, R2, L2, R2. A new cheat menu will appear.
Baby Soccer bonus map
During a game (before it ends), keep L1 + R1 pressed and do the same sequence: L2, R2, L2, R2, L2, R2. The names of the players appear on the baby globox.