How to make my 5.1 surround work with my PS3?

My PS3 is currently connected to my tv by a HDMI cable, the tv is then plugged into my amp/dvd player via a digital optical cable. I have fiddled with the settings on the PS3 for sound and so far have only managed to make it work through 2.1. Can anybody tell me what settings I should change to make my PS3 ready to output via HDMI in 5.1 so that I can eliminate it as the source of my problems?