Hi wmalik,
You will need to identify and test a few things.
Find control module, this often includes an integral speaker.
Does this have 12vdc present to module with ignition on?
The reverse gear switching method used and associated cables (perhaps to reverse lights), check this 2 wire link.
Further TestingIndividual sensors usually have 3 wires each,+vdc, neg, signal.
You can test these with multimeter but they are often sealed for weather proofing. Spike cables with a small dress makers pin point and hold multimeter probes on to pins to test.
Also you could try using a long thin screwdriver as a stethoscope, hold screwdriver tip on each sensor and listen through handle for slight vibrations/hum. This will tell you that the sensor is working and active.
If a given sensor is active, get an assistant to move a large spanner or similar object towards the sensor face while you measure voltage at signal pin and ground. The voltage (millivolts normally) should change in relation to the distance of the obstacle.This is the signal itself (sine wave) sending data to the processor to increase frequency of beep/sounder.
Suspect break in wiringYou could do further tests if you suspect a break in the harness/wiring once you have located the parking sensor control module as you can then measure for continuity between each of the 3 wires in each sensor with its partner connector at the module. To do this , set multimeter to continuity mode (beeper) , use a pair of 3mtr long pieces of wire and attach one end of each wire to probes to make an extra long test lead set. Use dress makers pin method at both ends to spike sensor and insert into connector at module.Measure for continuity at each of the 3 wires for all sensors around the car.
Suspect control moduleRemove from car, try and gain access to pcb of unit, check protection diode/s, check poor solder joints.
If you spend the day doing this and still no luck...give up
But you will have learned a lot about diagnostics without going to mazda dealer
Hope this helps.