The in-built microphones of many a camera leaves a lot to be desired, the quality varies from model to model, their small microphone diaphragm not helping any and favouring the high frequencies rather than the middle and low frequency end.
Sounds like too much microphone gain and lack of sound level, usually gets worse as you move away from the microphone, we really could do with thinking of it as sound pressure rather than sound level since it has to move the diaphragm in the microphone.
Some tape recorders shows this very well, on some you could adjust the microphone gain so that it doesn't overload the audio circuitry and on some it did this automatically by AGC (Automatic Gain Control) action.... loud signals would reduce the gain, low or weak signals would increase the gain, you could hear this quite easily on playback, the background noise increasing and decreasing between the sound.
I don't know the SX700, if it has a seperate microphone input socket I would try using a external microphone, I am sure the sound quality would be far more superior, depending on the quality of the microphone, you say you had three and they were the same, that would seem to be a good indicator.
Cameras ain't no video recorder, I've repaired camcorders with the same 'hissy' type sound... the small sized microphones are just a gimmick and there's very little quality in them, using them outdoors is even worse.
Dave
