Problem with Some Center Channel Audio from Blu-Ray Discs

Please don't laugh at the equipment I'm using.  I'm on a tight budget, and have cobbled together my home theater over the past 11 years as I could afford various components.  It's not much, but it's good enough for family movie night.

All that being said, I've run into a problem that I'm not sure how to troubleshoot.  Here is my list of components:

Receiver -- Sony STR-DE925
TV -- Panasonic TH-46PZ80U
Blu-Ray Player -- Panasonic DMP-BD605
Speakers (5) -- Cambridge Soundworks Ensemble IV

My problem is with some center channel audio while playing Blu-Ray discs.  I have noticed the problem with "Cars," "Ratatouille," and most recently with "Valkyrie."  It has happened with other titles as well.  Some voices create a sound that is difficult to describe, but I would best describe it as a muffled or garbled static sound.  I can hear what the person is saying, but it sounds like garbage.  In "Valkyrie," Tom Wilkinson's character, General Friedrich Fromm, was the only voice that had this problem.  All others sounded just fine.  I have swapped speakers around, since all 5 of mine are the same, and I tried a different spare speaker that I had available.  I checked all the connections, and they all seem fine.

This problem only happens with B-R discs.  I have never experienced it with standard DVDs or with HD content through my cable connection, both of which input into my receiver through a digital optical cable.  DVDs play back fine on the same B-R player.  Only B-R discs have this problem.  Because I have an older receiver, I have to input to it with digital optical in bitstream for 5.1 channel surround.  I have the video running to my TV via HDMI and the audio direct out from the B-R player to my receiver via digital optical.  If I mute my receiver and play the audio only through my TV speakers, I do not experience this issue.  My B-R player is on the latest firmware revision (1.7), and Panasonic technical support was no help at all (I think their call center is on the Moon, the reception was so bad).  My B-R player is set to downgrade all the new fancy audio formats to good ole Dolby Digital or DTS.  To be honest, I have no idea if this is an issue with the B-R player, my 16-ga speaker wire, the receiver, or my wimpy speakers, or some combination of them.  All I know is that it is extremely annoying and I have no idea how to troubleshoot it or where to even begin.

So, I turn to you fine folks, people who actually know what they're doing with this stuff, for guidance and ideas on where to begin.

Thank you in advance for any advice you can offer.

Thanks,
Jason
Minnesota