Yes you can. This is a nice feature if you have little children not ready to wear the headphones or the entire family is watching the same DVD wishing to hear the audio.
Also this is very convenient and does not require any additional cables or connections to your vehicle stereo system. This is accomplished using the built-in FM transmitter feature. With the FM transmitter, you turn the FM signal on using the FM-TX button on the remote control. For most models, the FM signal frequency will then be displayed on the headrest monitor. If you wish to dial to a frequency not being used by your local radio stations you can use the "left" and "right" navigation buttons on the remote control to adjust the frequency on the headrest. Once selected, dial your vehicle stereo system to the same frequency and you will hear the audio from the headrest over your vehicle speakers.
Please note, only one signal can be received at a time for your vehicle stereo system so obviously listening to 2 DVD's at the same time over the vehicle speaker system is not possible.