My doubts were funded on the design of my own cable, that shipped with XBox 360. It has two RCA for sound and one RCA for video but, in the XBox connector, it has too much plastic, in a way that it covers the HDMI connector.
Therefore, I supposed that that would be a design issue, with that purpose (to prevent to connect the two cables at the same time), and not a design flaw.
But then, I found the kit shown in the picture on sale: it includes a standard HDMI cable and a connector without that extra plastic, that can be plugged together with the HDMI cable, and deliver sound to two RCA (and an optical connector).
Already ordered the kit on eBay. Waiting for it.