Speech Not Transmitted thru Headphones, just Music

[cwoernle]

Audio from my Panasonic Blu-Ray DVD player connects to a Kenwood receiver. I get surround sound through the receiver's speakers when I play either Blu-Ray or regular DVDs. However, if I listen to a Blu-Ray disc via RF headphones hooked to the receiver, music is transmitted but voice (speech) is not. If I listen to a regular DVD using the headphones, I hear both voices and music. Can someone explain why this happens and if there is an adjustment I can make to get Blu-Ray disc voices to play in the headphones?
Thank you.