You can get an attachment to provide an optical-out that'll plug in to where you use your composite, though I think you can use both your composite for sounds + HDMI for video at once.
edit:
i.e. box shown in this: http://www.sheergadgets.com/contents/products/Xbox_360_HDMI_AV_Cable/Xbox_360_HDMI_AV_Cablelarge.jpg
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