Here's how to turn the seat belt chime off, so do the same procedure and it should turn on. Regarding the beep when you lock the car - dunno, maybe it's a dealer changed option (and it's not the horn that beeps but some small chimie in the driver's side fender area.
1. Sit in the driver’s seat with the key in the “ON” position. Using the “display” button on the steering wheel, rotate through the displays until the LCD on the gauge cluster is only reading the mileage, not trip A or B, just the mileage.
2. Turn the car off and pull out the keys. Open the driver’s door and close it again. Put the keys back into the ignition and put it to the “on” position again. As soon as the LCD display on the gauge cluster goes from “hybrid drive” back to the mileage, begin to hold down the “trip reset” button to the left of the steering column for approx 30 seconds.
While still holding down this button, (it may be tricky) pull down the seatbelt and buckle it. At this point look at the LCD display on the gauge cluster. You may let go of the “trip reset” button at this point. The display will now read “seatbelt buzzer ON”.
Press the “trip reset” button once and the display should change to “seatbelt buzzer OFF”. You’ve done it! You can turn the car off, as the setting is now changed and will remain this way until you change your battery again, or repeat the steps to change the setting yourself.