you can just buy the official HDMI AV cable. it has HDMI lead + a “breakout” lead for stereo phonos ( and optical as well if you need). You may need phono to jack connectors etc to get to the PC speakers
Or if you are skint then just rip the plastic cover off the grey av cable that came with the xbox this will allow you to plug both the HDMI cable and the AV cable together thus you can then send the stereo phono from the AV cable to the PC speakers and still have the HDMI to the monitor. Again You may need to buy some phono to jack leads and a double female connector
If your PC speakers dont have a volume control then you may need to look at putting a little headphone amp or similar between.
just found this on this very site which you can see the phono outs as well - check it out.
http://www.avforums.com/forums/xbox-360/982786-hdmi-optical-out.html
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