Ah - I changed the sample rate in a different way - I suspect you've done it correctly. I will fiddle about later.
As to the filters - the code in analog_microphone.hpp says "
// don't ask, I can't remember how or why this works ... dick with this at your peril
enum {
COEFFBITS = 13,
"
So I can't at all think why that comment suggests it might be hard to understand or difficult to modify ;-)