You can't. You just need to turn down the volume on your TV. Audio will always be transmitted from the 360 over HDMI. I'm currently using optical and the sound still comes over HDMI to my monitor.
If your issue is that you're not getting audio from your 360 via optical, check the codec settings in the dashboard to make sure you're not using one which is not supported by your sound system.
This post has been edited by Aldanga: Mar 6 2012, 09:57 PM