intuitively, it sounds like a setup issue, but:
1. with the digital connections, are you currently getting sound through your 2 surrounds or only your fronts?
2. exactly what audio mode are you using on your b&k (surround, thx,dvd audio?).
1. try connecting only 2ch rca analog from your bd/fios to your b&k analog inputs and playing in surround mode. do you get center info?
3. are you connecting the fios to the tv input on your b&k? the manual says "if you have source with only digital output don't connect it to tv"
4. are you having the same issue with digital coax as you are with digital optical?
5. have you tried disconnecting the hdmi cable from your bd/fios and playing audio only? what happens?
6. in your current set up, have you tried connecting one of your speakers to the center? what happens?