Too hot to do much outdoors, so I am hanging around the computer and picked up on your return already. Thanks. Your thinking is right along where I have been at, as far as the SPL thoughts.
To start with, will be using an old battery cassette machine. With a little circuitry, I can have it turn on and off with sounds that are picked up As far as a little AGC, if needed, I will probably just use an averaging peak detector, and drive a LED into a photoresistor. Something very simple that I can play with, yet get good results. Will be putting in another Op Amp and can put the LDR in the feedback circuit very simply.