You need to check the plug behind the DVD Unit:
There is a TSB on this exact problem:
Honda Service News December 2004
No DVD Sound From the Wireless Headphones
2005 Odyssey EX-L or Touring model
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.