Hi,
Did you flash the firmware for 9600 baud rate ?
As the documentation points:
The examples use the SoftwareSerial library to communicate with the Ping device, which allows flexibility for the rx/tx pin assignments on the Arduino. The examples use pins 10 and 9. The SoftwareSerial library can only communicate with the Ping device at 9600 baud . If you are using a board with more than one HardwareSerial port, like the Arduino MEGA, you may use the second serial port to communicate with a Ping device at 115200 baud.