I got Panasonic 42 viera plasma tv but there's no audio out on the back How am I going to use my Philips home theater sound system?

The proper way for a home theater to be set up is for all audio and video input components (DVD Player, Cable Box, etc) to be connected directly into the home theater receiver with the only video output being the the TV. The TV should not act as an input device to the receiver, only as a output monitor. The sound being produced by the components will then go directly out to the speakers. If you are trying to set up your components directly into the TV and then feeding the TV into the receiver, you're setup is backwards.