If Canon isn’t going to do this stuff, others appear very motivated to do so.
“As an early goal, I have disabled the AGC for on the AK4646 audio chip that the 5D uses by overriding the sound_dev_task() at 0xFF856E60 to call audio_set_alc_off() at 0xFF856B14. The level is much too hot due to the wrong levels selected (it defaults to max gain?) but there is no shift in quiet scenes anymore. This is an easy first step that provides a proof-of-concept modification for the 1.0.7 firmware.
I can also log the audio levels as they are being sampled, which will make it possible to do on-screen level meters at some point in the future. My next step is figuring out how to add menus for the AGC disabling, then tackling the MVR_start_record() and LVAE_* functions.”
Hack in progress