sorry the radio transmitter idea isn't going to work well unless you have a really huge yard. Even with a very low power transmitter, and I mean in the low miliwats range, you're going to hear them too far away, and direction finders just don't work that close to the source. Besides which who wants to carry a receiver and mess with it all the time.
I think wiring up the speakers is going to turn out to provide the most reasonable answer and surve other needs too. tom