Hi Batman,
For your microphone to be able to pick up a normal, soft voice from about 15 feet away, it must compete with phones ringing, dogs barking, TV, etc. from the next room. If you really want such a high sensitivity then the mic preamp (electret mic) must have a voltage gain (not power) of a few hundred. A low-noise opamp or two will do it and the feedback resistor can be variable to reduce the sensitivity (bull-horn?).
For a variable frequency pickup, it needs a complicated variable bandpass-filter circuit.
Google "VOX circuit" for schematics.