The PS3 offers user selectable audio output utilizing one of three different connection methods.
1. HDMI - Just use an HDMI receiver... You don't have this, so you can't do this! If you are connected to the display via HDMI, then it will default to HDMI audio and you will manually need to switch to a different audio connection type.
2. Toslink digital optical audio... This isn't HDMI, but it's a solid audio connection. You will need to go buy this cable.
3. Analog (red/white) RCA audio connection... If you only have this on your receiver... maybe time to upgrade. The RCA connectors are on the proprietary PS3 cable which shipped with your PS3 unit. Plug that into your PS3 and go into the audio setup and select the RCA connections. They have a pretty picture and everything!
Only one audio output may be active at a time, but it definitely works when you set it up for your type of connection.