Here is a small two transistor FM transmitter that can transmit up to 1/4 of a mile. That is a good distance and should not cause many problems interfering with other radio communications. There are a wide variety of applications for a small device like this. You can build one to listen to nature sounds. You can put this in a waterproof housing and put a 9 volt battery in it. However, after a while, that is going to drain your battery down. For a listening device that kicks on when sound hits a certain threshold, you need a small sensor that can pick that up. Almost like a photo sensor.
Here are the list of parts:
R1,R4,R6 = 10K C1,C2 = 0.1uF Q1,Q2 = 2N3904
R2 = 1M C3 = 0.01uF L1 = 0.1uH
R3 = 100K C4 = 4-40pF
R5 = 100 ohm C5 = 4.7pF
R7 = 1K
I would recommend buying your parts at Digikey or Mouser. Both are good parts suppliers. It all maters how low of a price you can get.
(source – http://www.uoguelph.ca/~antoon/circ/fmt1.htm)