Let me make sure I understand what you're saying. You have the 3 composite cables from the PS2 plugged into your TV and have the optical out from the PS2 connected to the cinema system? If you're TV is the same as mine, it doesn't feed sound from a games console back to the cinema system (why I don't know), so you can't use the TV Sound mode to listen to the PS2. If you're playing your PS2 with the cinema system in digital aux mode then it will ignore the sound from the TV anyway so no harm will be done.
For my PS2, I have only the video composite plugged into the TV and the sound plugged into the analog AUX of the cinema system but the sound is weak. I will try using the digital out instead.
It seems to me that if you want the sound from the PS2 to come through both the TV and cinema system that you need to have it hooked up the way you do.
The problem I have with this is that I want to have a "spare" optical cable plugged into the cinema system that I can plug into my MP3 player whenever I want. Alas, there is only one digital-in line so if I have my PS2 plugged into it I will have to keep switching the cables

Is there any way round this?
I will hook up the PS2 through digital aux tonight and test with non DD/DTS games to see if I have the same problem. Will get back to you about this.
Cheers,
Dave