It may be that the movie wasn't encoded in a 5.1 format. Therefore, depending on how you have your surround sound system set, it may just be sending the audio to four of the speakers (common with Matrix Mode). If you're still getting 5.1 with DVDs and Blu-Rays, then it's likely the streaming.
I have a Samsung BD1250 I have 5 mbps for internet speed when downloading. I just set it up & downloaded the most recent patch (sorry not sure what that was) two weeks ago. When I try to stream a movie using Netflix I get sound through all speakers but the center speaker. When I play a movie using a DVD the voices are heard through the center speaker. Has anyone else run into this & is it an issue with Netflix or the Samsung product?