Hi Rock Liu,
Can you please let me know if there is any issue with the internal mic ? Is there any noise reduction ON by default on the Firmware itself ? I was checking the videos recorded by the C5 and all of them have a really bad sound that goes sometime up and down with a kind of constant feedback noise on the backgroud.
Please check the below videos and the difference in the noise during the video recording. The C5 is actually under the seat with a small rubber pad to avoid any vibration.