For lethal sonar, enter any Unlimited Air code, then press A + B while playing. Your sonar will look the same, but will kill all enemies it detects.
Go to the powers of levitation level and find the room where the Clan General is.One wall moves - swim through that side and you'll find a football on the floor near one of the lights. Pick it up, then go to the 'level select' screen and you'll find an option for 'Dolphin Soccer' at the bottom of the list.
When you are swimming, turn Ecco and pause when he is facing forward. Then press:
Right, B, C, B, C, down, C, up
This will open up a menu from the top. From here you can turn invincibility on and off and select a level (NOT in chronological order, and some numbers go into glitch levels).
When the level name appears, hold A + Start until the level begins. When the game screen appears, unpause and you'll be invincible.
To turn off the background music, pause the game and press A five times.