You have two options:
- HDMI Audio Splitter - a box which plugs into HDMI, and outputs HDMI (for video) and L/R RCA (for audio)
- Optical to RCA adapter - a box which plugs into SPDI/F out on your XBOX via optical cable, and outputs L/R RCA. This option seems to be cheaper than the other one.
From there, you will need another cable/adapter to route the RCA audio to whenever you want it to go.