All 99 mudokuns:
At the main menu hold R1 and press triangle, square, up, up, down. 
Sent in by BKA759@aol.com

Level Select  -  At the main menu, hold R1 and press Down, Right, Left, Right, Square, Circle, Square, Triangle, Circle, Square, Right, Left.

Movie Select  -  At the main menu, hold R1 and press Up, Left, Right, Square, Circle, Triangle, Square, Right, Left, Up, Right.

Green Air Mines  -  During play, hold R1 and press Up, Left, Right, Square, Circle, X

Instant Voice Puzzle Solution  -  During play, hold R1 and press Triangle, Up, Circle, Left, X, Down, Square, Right

Secrets 1 - is on the very first screen of the game. Get yourself behind the barrel and press down. You will need a grenade to successfully complete this secret so you'll need to come back after finding the boom machine.

Secret 2 - is the big barrel next to the sleeping guard. When he wakes up possess him. Make him run to the right until he can kill the other sleeping guard by the elevator. Yell "look out" for extra fun then waste him when he wakes up.

Secret 3 - is where you pull the lever to open the trap door and make the Slig fall in. After you dispatch the Slig pull the lever again and jump into the hole before the trap door shuts. This is tricky and may require several attempts.

Secret 4 - is where you pull the rope handle. Do the same thing and jump in the hole before the door shuts. Another fast and tricky jump.

There are more, but you can now find those since you know how the game designers have hidden their secrets. Some other hard to find Mudokens: There are two workers hiding in the shadows you yourself hide in. When you are completely hidden in each shadow say "hello." If you hear a reply then say "follow me" and lead them to the nearest portal. 

Near the long elevator at the end of the level go to the top with a grenade. Get in the middle of the ledge and crouch and throw grenade to the right. That should kill the guard sleeping on the small ledge.

Ride down to make sure then go back up and jump off to the right. You'll land on the ledge. Duck and roll carefully into the next room to avoid the grinders. Standing jump for the first one and a timed roll through the other two. Continue rolling to the next screen and free the last three Mudokens.