yes, if you use the dash add-on that attaches at the top of the dash for the carpc screen, you should be able to use the aux-in to supply audio to the factory stereo.
with this method, everything in the car will work exactly as it does with a mp3 player-- ie: your steering wheel controls will work for most functions, but you will not be able to change songs, or other things on the carpc using the steering wheel controls.
if you choose to replace the factory stereo with the carpc, you might be able to get the steering wheel controls-- it depends on how they are setup-- there is some devices that might be able to decipher the signal from the steering wheel controls to allow them to control the computer, but this is very broad-- without knowing your car, i can't say if it will or will not work..
also, the way that you would connect the carpc to the speakers might also need extra parts if you remove the factory radio.
if your car has a amp that takes rca, or low level audio inputs, you should be able to connect to it with just a couple of adapter cables.
if your car uses the factory deck to power the speakers, you will need a amp--amps range from $15-$200, depending on model, and power rating..