The original head unit would have had a dedicated input for the DVD player. I would check that the cables on the rear of the unit are still connected (under the rear seat) and that the input to the head unit is connected. If so, test the head unit by connecting a known working audio source to prove the input. If you still have the head unit and the wiring has not been modified, put the original one back in then verify that everything works. Could be a cable to input mismatch.
With the sensors, my estimation is that they hijack the front speakers (they are possibly in parallel with the speakers).
Remember that there are 12 speakers in the car. If the new head unit does not allow for the full array of speakers the ones that were originally used may not be connected anymore. Play some audio and check for any sound in the individual speakers ( will be tricky unless you can isolate or remove individual speakers to hear them or use a meter to measure resistance/voltage off the back of them).
I would get the head unit installer to verify that the unit is actually compatible with the setup of the car (bit late now perhaps). Does the rear control and the steering wheel control still work?
I'm currently investigating a car kit with bluetooth and iPod connections and from what the dude tells me there may be some modifications required to steal the audio output (taking the speakers to allow for the phone and iPod by cutting the original head unit output - when the call ends or the ipod stops, it returns control of the speakers to the head unit)- at this point he is researching if it can be done or if it requires additional hardware to keep all original functions working.