If you want to compress the range of 0-30 kHz to something like 0-12 kHz you can do that with a switched capacitor delay chip like the Panasonic MN3007.
It will work like the bat detector, except it won't need to clip and threshold the audio. You will need to use a slow ramped VCO (continually ramping the sampling frequency down) in order to do this. There are some projects at:
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You may be able to adapt a flanger, for instance, to suit your purpose.
The control voltage to the sampling VCO will be a sawtooth wave. Unfortunately, you will hear the sawtooth period as an artifact in the output. Maybe you can filter it out.
I guess an all-digital solution is better ;-)
Frank Raffaeli