No Surround: PS3 (LPCM) and Sony STR DG720

[ueberbill]

I've hooked up my ps3 with an HDMI cable to my brand new Sony STR DG720 which is my first HDMI-switching receiver so I was picturing ear-melting surround. The problem is that I can't get any. The ps3's audio setting is set to auto and the STR sees the audio coming in and automatically opts for Linear PCM, 48 hertz. It sounds great out of the fronts, the center, and the sub, but I get nothing out of the surround speakers. I'm not going 7.1 initially, just trying to get the plain ol' surrounds working. But nothing- the menu options around the surround speakers are greyed-out. The test tone feature of this receiver has plays sound of both surround left and right so I know they're hooked up correctly. Is there something I'm missing. I'll be happy to provide more detail if needed. Thanks a lot.