I've purchased some DVD headrests for my car and I'm going to connect them to my AVIC-8000NEX using the rear a/v out cables, but I was wondering how the head unit plays audio for the rear units.

For example, if I select "DVD" for the rear output, will it also play the DVD audio through the rear speakers in my car while allow music (SiriusXM or HD Radio) on the front speakers?

The head unit has rear outputs for video and audio. Head phones have to be hooked to rear monitor or the monitor would have to have its own speakers. 

I have two monitors and had to use splitter for each channel because each monitor has its own rca plugs. I purchased a video splitter/ amplifier because i read the video signal is weakened when split. Hopes this helps.

It will not play dvd on the rear speakers and siriusxm on front speakers, the only audio that plays different from the front and rear, is the aux audio output that corresponds to the aux video output. 

Thanks for the information.

I've connected my DVD headrests and I can confirm what you said.  The speakers in the car only play the "main zone" audio but if the rear a/v unit (i.e. my DVD headrests) supports it's own audio output that will work.  (i.e. headphones)