Having sound off or on will not gain you any real use able extra file space, just like having HDR on or off will not gain you any use-able space. The only thing that would take more or give more use-able space is if you flashed a higher or lower bit rate firmware.
I would leave it on so that you can call out plates the camera might miss if you are in an accident, and turn it off or use Registrator Viewer to mute it if you are doing a review or posting and do not want conversation herd.