You need to check the plug behind the DVD Unit:
There is a TSB on this exact problem:
SOURCE:Honda Service News December 2004
TITLE:No DVD Sound From the Wireless Headphones
APPLIES TO:2005 Odyssey EX-L or Touring model
SERVICE TIP:If the digital audio 2P connector from the DVD player unit to the audio unit is loose, or it has an open or a short, you'll wind up with this situation:
- You can't hear any DVD sound through the wireless headphones, yet it comes out through the speakers OK.
- You can hear the radio through the wireless headphones.
- The RES display works normally.
To fix this problem, make any needed repairs, and make sure the connector is properly plugged into the audio unit.
Turn the rear entertainment system ON.After the main power switch "VOL PUSH PWR" is turned ON, push "REAR PWR" switch to turn the rear system ON. To turn the rear system OFF, push "REAR PWR", and to turn the whole system OFF, push "VOL PUSH PWR".
Be sure that someone isn't inadvertantly hitting the Switch or stopping the DVD.Also, be sure the Batteries for the Headphones are Good. Some will shut off if there isn't a signal after 30 seconds.They are having different problems with the DVD, but no Recall or Total Fix.