Plugging in HDMI Disables Output Audio on Headphone Socket :(

This is a long shot.

I output the video display via the HDMI port to my projector but, this disables the audio output of the headphones/speaker sockets! I want to feed the audio from this socket to my surround sound amplifier/processor.

Does anyone know a way, or is it possible to configure audio to be output to the headphones/speaker socket when an HDMI cable is plugged in?

Cheers