External microphones connected to the HERO2 take approximately 5 seconds until they are properly recognized. If you start recording right away after connecting your external mic to an already powered on HERO2, the audio will be recorded with mono audio. You may also notice a "tearing" sound at the beginning of the clip.

The best practice to ensure that this does not happen is to follow this procedure:

  1. Power on the camera.
  2. Insert the external microphone.
  3. After about 5 seconds have elapsed, begin recording.

This will ensure that the audio is recorded in stereo.