Either the player - or the disc menu - has selected 5.1 audio. If this happens, the center-channel voices don't come through. Try to set everything to stereo, and you should get a satisfactory result.
Bitstream wouldn't work, because then the TV has to decode the sound and downgrade it. Definitely try PCM, and set the output to Stereo both on the disc and the player.
I have PS3 connected to tv only using tv speakers connected through HDMI cable. When i tried playing movie yesterday the background sound works fine but there is no speech when people are talking...... Tried to play with audio decoding putting it to PCM or bitstream nothing helps.......