put one of those disney blu ray discs and go to the home theater setup routine, or find a similar disc. it will allow you to put static noise through each speaker location individually as a test. just a few thing to consider:
PS3 audio output settings. I don't have a PS3, but I imagine there are options.
As someone mentioned, is the sub working? Are any of the orange lights blinking (either on the bar or on the sub) if so, they are not communicating and you will need to re-sync according to the instructions. And then a power cycle as well.
assuming your led volume lights are still working, get the remote and adjust the volume for each channel and make sure one of them isn't way down.
check the rear speaker connections. I found that my cables had come out of the speaker when I had an issue. just remove the plastic cap on the rear speaker(s) and check that the termination is tight.