Why won't my Blu-ray discs play in surround sound?

[inlayterms]

Hi. The "Blu-ray and PS3" and the "Blu-ray Technology and News" forums didn't seem appropriate, so I started this thread here. Please feel free to suggest if it should be moved.

My problem is that I can't get my PS3 to play Blu-ray discs in 5.1 surround sound, regardless of whether I connect my PS3 to my TV via either HDMI, or Component, cable. Now, in either case, I also have my PS3 connected to my A/V Controller via a Digital Optical cable and, when I am playing either a PS3 game or a regular DVD, my A/V Controller outputs sound in 5.1 surround sound.

But, when I play a Blu-ray disc, my A/V Controller will only output the sound in either Pro Logic or 2 Ch. My PS3 will not play Blu-Ray discs in 5.1 surround sound. My A/V Controller displays the 5.1 (and corresponding Dolby Digital or DTS) icon when I am playing either a PS3 game or a regular DVD, but only allows me the option of Pro-Logic or 2 Ch. when I play a Blu-Ray disc.

Does anyone know why I can't play a play either a PS3 game or a regular DVD in 5.1 surround sound but not a Blu-ray disc? If so, care to tell me how? Thanks.