Dear scream_er
Hi
Sure. Remember : if you use an op amp as a simple comparator ( without feed back) if the v+ be higher than v- , the out put will be about +vcc . and if the v- be higher than v+ , the out put will be about -vcc . you can use a simple capacitance microphone , near your baby , and give the out put signal of that , to the voltage amplifier , and then give it to your op amp( its v+) and give a variable dc voltage to the v- of that (with potentiometer , to have calibration as well as ) . if the microphone get the sound of baby ( when you are not near your baby ) the v+ will be higher than v- and the out put will be about +vcc , and then you can excite the base of BJT to give command to the relay , and then the relay can turn on the alarm system!
Best Wishes
Goldsmith