Seat belt remind sounds feature works when already belted
 

Hello,
i have the 2012 ct,
today I have noticed that my car doing the sound reminder for the seat belt when I’m actually already belted.
its only doing that every time that I’m braking.
Please help me with that guys
thanks.

First thought is just a loose connection and the car momentum when braking causes it to the connection to break. I'd say you have 4 options (since you know you are belted and safe ;)
1. Ignore it
2. Look for a loose connection or broken wire. I'd start under the seat and work forward...but also sounds like more work than the next option.
3. Get an OBD2 BT device and you can use Dr. Prius App to turn off the beep (not sure if you can make that change with Techstream)
4. Take it to a dealer.

I'm not the greatest at putting on my belt right away (which drives my wife crazy), so I got the Carista OBD dongle and just turned off the beep. Now she is not reminded that my seatbelt hasn't been fastened yet.

Could be the receiver or the wire that connects to it. You could remove it, take it apart, and investigate it. Or start simply by checking for damages to the receiver itself or to the wire. I don't know the exact setup but on prior cars i know there's a wire that is down by where the receiver is. maybe someone jammed a vacuum cleaner down there and knocked it? worth looking at at least! start simple first and work your way from there. Look for debris inside too.