using a two channel amp/pair of fl/fr speakers w/o center speaker, you will need to go into each movie disc and find the audio settings and change it to 2 channel stereo.
they default to some surround version, usually dolby 5.1. In those default cases, all the dedicated center channel information will be missing (totally). Much voice info will simply not be present unless you tell the bluray player to send the center info to fl/fr.