You may not be able to remove the nerves, and making calmness the goal can give you one more thing to fail at. Give your attention a more useful job.
Before the audition, reduce avoidable uncertainty. Know the instructions. Prepare the scene. Test the technical setup. Decide what you need from the reader. When you begin, put your focus on changing the other person rather than monitoring whether you look nervous. If your body is activated, let it be activated. You can still listen, pursue an objective, and take direction with a fast heartbeat. Build a short routine you can repeat under pressure: breathe, name the first circumstance, connect to the reader, begin. Confidence is pleasant when it arrives. A repeatable process is more dependable.