I think I once looked into this a tiny little bit (for my a570is) and didn't find an obvious way. But that was a long time ago, we might know more now. Anyway, Canon uses/has used several different audio chips, each having different features and different control logic. This means that learning how to disable AGC on one camera probably only helps a couple of other models with similar audio features that were in production that year.
Anyway, the a570is firmware has a string 'AudioIC_LA74310LP.c' implying that its audio chip could be the Sanyo LA74310LP 'Audio Interface for DSC + Video Driver'. This chip has a built-in AGC (ALC) feature for audio input. The datasheet I have isn't really verbose on these things (or anything else for that matter), but it does show a startup logic control example which at one point enables the ALC. But I'm afraid it probably enables the ALC amplifier which is always in the signal path, and disabling it would mute the mic input.
Another way to disabled AGC would be to take the camera apart and alter the audio circuit e.g. by doing something evil to pin 6 which is the ALC detection pin. That wouldn't give you manual gain control, though, just fixed gain that's probably no good for all purposes.