Above water the camera audio will be muffled and low. Sound travels through air but since the camera is waterproof, air and sound waves can not get through the housing easily. The sound will thus be muffled/low.
Under water the audio/sound will be much clearer. This is because water is much more dense and sound waves can easily travel through the housing.
If you want better sound above water you can alternatively remove the end cap - just be careful around water!