AFAIK CHDK does not support this. And this might be also hardware problem, I'm not sure, wheter such microphone, as used in camera, is able to manage such loud voice...
In general, CHDK helps little with video. It support quality override (mostly decreasing of the quality), manual zoom/focus if camera doesn't have them.
Also, in general, CHDK does not support anything that is related to the sound*. We are not able to read microphone input, record sound-only, change sound volume and so on.
*To precisize: nothing related to the sound, excluding firmware beeps, shutter click and simmilar sounds;) Rather not usefull in video recording itself.