Seeing as you want to know when YOUR baby is crying, you should record him/her crying so you have a repeatable cry to tweak your filters that you have not yet implemented so the gadget you are building does not put your spouse in hommicide mode with you as the unlucky victim.
To disambiguate my last statement... simply selecting a level for trip will drive your wifeee insane with nuisance trips from any noise....albeit the lawn mower next door, the front door opening and closing, ect. Filtering before the trip detection will allow you to tailor the heuristics of your preamp to match that of your baby's cry, thus making it less sensitive to ambient noises you don't want trips from. Such as dog shock collars are filtered for the unique content of a dog bark so the poor animal doesn't get a jolt when he scratches his head. (I owned a cheap one, it couldn't discern the clank of the d-ring from a bark, so the dog pretty much hid under the bed whenever the collar was on..)