I too have been interested in bat call detection. Bertrik's pages and info are very good. 10 years ago or so, when I got interested in this, Knowles had invented the Sisonic mircophone. It was a freefloating silicon microphone. Very small, like a matchhead. Invented specifically for the cell phone market.
They had some that were broadband i.e. *flat* response to from ~1khz - 100Khz, with some wiggles going to 200KHz. Awesome unit. cost about $6.50 at Digikey. I ordered some and made a mouse microphone for a researcher. These would be awesome for such a project. Some were straight mics and some had integrated amplifier/buffers.
Unfortunately, looking over Digikey, I only see units rated for 20 Khz. They might have extended response. I had emailed and called Knowles about the mics I was interested in. They sent me a proprietary set of data I believe - showing the wiggles.
I don't know if the mics I ordered still exist or even what number they were. Maybe there was no market and they were taken out of production? I couldn't say. They were a fantastic product, and could possibly replace specialty mics that were $100 but needed a $1K - $3K preamp!
All bells and whistles would be a high sampling rate unit 100-200 KSamp/sec rate followed by a mathematical transform that down shifts the carrier frequency to the human audio band, (and does time expansion) while preserving the harmonic content. One Flemish EE Prof. described such a project, but he got married and changed his name!
I don't quite have time this week, but will definitely look around to see what happened to the Sisonic unit.
Bat detector folks are unfortunately a niche market!