Related: I wanted to connect some crappy PC speakers to an Xbox 360, which has a weird-ass AV connector on it (as well as HDMI), but it does also have digital optical out.

I got one of these ("Optical and Coax Digital to Analogue Audio Converter") from Maplin. Yeah, it's Maplin, and it's probably overpriced, but we were in a hurry, and it does seem to work.