Get a TI RF4CE chip. cc253x. Very low power. Cheap, ubiquitous. Devkit $200.
Get a micromems microphone. And a few discretes. And a tiny battery. You gotta bug.
Probably sucky antenna since you're size constrained, but deal with it.
Use expensive, small components. While you could build this for $20 as a hacker, your budget as a spook engineer is a few grand per. Live it up. Size matters. You can use as many signal layers on the PCB as you like. You can use exotic PCB materials if it helps. Shit, you can custom integrate an IC and have your fabs make 'em.
Store audio, compress, encrypt, transmit when "read" command received. Only wake up periodically (or time based pseudo-randomly!) to listen for it. Do this on minimal power, for as long as possible.
About 20 mA to listen 30 mA to transmit, for maybe 20 milliseconds each. Can't escape that much.
Actually since you're a TLA you can integrate all this on a 1x1 mm IC, size dominated by battery and whatever you use for antenna. Integrate since you own some nice secret fabs.
Of course the antenna in the white fiberglass truck on the street is very high gain.
To compensate for the mote. Also I suppose you get advanced battery chems, though not RTGs :-)
And you probably need to integrate a lot more audio storage flash on the IC; easily enough done.
With really small batteries you could be getting close to motes. A photocell and a an integrated supercap could last for a long time, but how much energy does it take to record and compress intermittent audio? And burst-encrypt-transmit cycles?
Interesting how, um, low energy *audio monitoring* approeaches physical limits as we get better, much like computation.
Yeah, I'm sure the TLAs could do this years ago. But now, everyone can.
ISM make me so horny.