Screen mirroring audio stutter

That will occur when there is an issue with the connection.

Make sure no other activity is occurring over the network (other streaming, web use, network backups etc)

To see a current status of the network run a report using istumbler or similar. Look for signal strength, noise, nearby networks

If Apple TV is on wifi try using ethernet

What are the values for signal strength and noise? Did you check for nearby networks? You may need to change the channel on your router. Distance between device and router will also be a factor, it is strongly suggested to try ethernet as part of the troubleshooting process. Did you ensure no other activity was occurring on the network?

When mirroring it is sending out the full display in real time so it will be sensitive to any inconsistencies in the connection as well as other activity. In those events it will not be able to keep up and result in lag/stuttering.