I know this is elementary, but did you check the settings and see if it is adjusted (faded) all the way to the rear speakers ?
If it is adjusted to centered for both front and rear speakers, see if adjusting to all the way to the front speakers does.
If you still have no sound, pull the stereo head unit and see if the factory harness is connected properly. If there was a previous owner, they may have butchered the harness to install a custom unit & just rigged the original radio up to the rear speakers to have some sound to sell the car to you.
If everything looks good behind the radio, get a wiring diagram and tap into the wires on the vehicle side of the harness plug for the front speakers and see if you can get some spare speakers to work from there.