The HERO cameras have two different sound settings, HI and LO.  Whenever placing the cameras on a surface susceptible to heavy vibration such as vehicles, the LO sound setting is required to avoid audio distortion due to vibration.  When placing the camera on more stable surfaces such as a helmet where the body absorbs much of the vibration, the HI sound setting can be used.  

On the LO sound setting it is normal to get some background electronic "fry" noise.  This however should be overwhelmed by the sound of the application the camera is being used for.  There are certain applications, street cars especially, where the engine or exhaust note may not be loud enough to get the desired audio on the LO setting.  One great trick to get higher audio levels when placing the camera inside a vehicle, is to remove the white rubber o-ring seal.  This will allow more sound into the camera's microphone.  This can only be done when using the camera inside a vehicle where wind noise and waterproofing is not an issue.