you would get audio for certain things and not for others.
I can tell you that you are best served by running an optical audio from the PS3 directly to your receiver and also one from the cable box to the receiver as well.
Chances are that you are not getting surround sound from your setup now as almost all TVs will not output 5.1 audio.
If you hook up your equipment separately using an optical cable for each, you will not only actually get surround sound, but my guess is that you would fix your audio issues as well. Just make sure that you set the PS3 to output audio via optical in the settings menu.
Not familiar with your AVR. I can only assume that you have chosen to hook your equipment up this way because you only have one optical audio in. I can tell you one thing for sure though, the way you have it set up is far from ideal. That's why receivers have multiple optical and coaxial audio ins.
I have a PS3 hooked up to my Philips 42PF7320A/37 TV via HDMI cable (also have my cable settop box doing the same). I'm then running optical audio from my tv into my Harmon Kardon AVR110 receiver, so that all audio comes through the same channel passing through the TV.
When I play DVD's or games on the PS3, the audio and video both work fine (ditto for cable). However, any downloaded content on the PS3 or CD's yield no audio. If I try downloading a trailer on the PS3, I get video but no audio. If I play a CD, it recognizes it, shows me the tracklist, and the visualizer reacts, but there's no audio.
I've gone through all the troubleshooting I can imagine and checked all the settings multiple times. Anyone have any suggestions?