Do you have a receiver and speakers currently or just some stereo miniplug speakers? Or do you currently own no speakers at all?
Receiver + speakers = Xbox -> HDMI -> Receiver -> HDMI -> Projector with the speakers connected to the receiver. If no HDMI, than component + optical to receiver then component to projector.
Basic low end computer speakers? Plug them into the projector if it has a sound output and hope it'll do any sort of audio pass through from HDMI or Optical or good old Red/White left/right RCA.
No speakers at all? Well, you'll need to get some then.