Not heard or capable of being heard.
The coach spoke to the young gymnast in a low voice that was inaudible to the rest of the team.
Inaudible adds the negative prefix in- to the adjective audible and turns it into its opposite. Modern spy technology (if movies like Three Days of the Condor or Patriot Games are accurate) can turn inaudible conversations into audible ones with the use of high-powered directional microphones. So if you think you're being spied on, make sure there's a lot of other noise around you.