PS3 optical audio problem

[actionhank]

I bought my PS3 2 months ago together with the $14 optical cable from gamespot. After connecting it to my Onkyo TX-SR601 receiver everything went fine. But then after a week the sound of the voices in DVDs, Blu-ray and Oblivion went missing. In Oblivion however the voice only disappears if you are talking directly to the character. In DVDs and blu-ray the voices of director commentary are there but not character voices during actual playback.

I tried everything including multiple restarts and checking the cable connections but nothing worked. Finally i bought a $60 optical cable from best buy and the moment i connected that, all the voices came back and was again listening in 5.1 surround (tested by turning a character around in Resistance Fall of Man)

But now the voices are gone again! I don't know what to do. I am not sure if its just the voices or if the center speaker itself is dead. But while watching cable the center speaker is definitely working so its not the connections to my receiver to speaker. I am thinking about buying another cable again and testing that, but does that mean i have to change cables every month?!

I have set everything correct in sound settings
optical>dolby digital 5.1 and dts 5.1

Also I set the dvd and blu-ray setting to bitstream

I also set my receiver to auto change into Dolby Digital, which it does whenever i load up a game or play a dvd. It switches from PLII Movie to Dolby Digital EX.

I output video via component cables

Oh and switching my receiver to stereo will bring back voices. Thats why i think its the center speaker

Any help will be greatly appreciated. Thank You