How can I fix the sound to the DVD palyer installed in my 2003 GMC Envoy?

I have a DVD player installed in the car the Screens are on the back of the headrest and the DVD is located on the passenger side under the glove departmeent. The funny thing is the tv's work but you can't hear any sound at all. I am going for a long drive and its going to miserable without a working DVD player for a 4 year old.