"Performance Sound" is an option that added an adjustable subwoofer to the stock stereo setup. Since by the time Performance Sound came around it was an add-on, GM/Pontiac just kind of placed it in the most accessibly and easy-to-throw-into space of the car - under the passenger-side of the dash. If you simply look in the foot well area in the passenger-side, you should clearly see the subwoofer protruding from under the dash.
If the prior owner installed a new stereo into the car, then there is a high likelihood the provisions for Performance Sound were not re-installed into the new stereo correctly or something else was done to prevent it from working correctly.
