Could be a few things. Check your sound settings on your XBOX and put it as Stereo (since you TV is probably only capable of playing in Stereo). If your redirecting the sound into your Home Surround sound system, then you'll have to check the manual of your sound system and go from there.
This could also be a faulty cable. This mat also be the beginning of a hardware issue. I repair xbox's and you can sometimes get them with no video and sometimes no sound. I believe this to more than likely be a faulty cable or bent/defective video port..