RasPi Stereo I2S Mic Hat!
If you need to get quality stereo recordings over a long period — say, for monitoring a mechanical system — then something like this Stereo Mic I2C Hat for the Raspberry Pi would be a great choice! With two INMP441 microphone elements connected to the Raspberry Pi’s I2S input, you get 24-bit audio from 60Hz to 15kHz with good sensitivity and SNR. Add a large SD card, or stream the audio to a NAS, and you can record almost indefinitely!
The INMP441 microphone elements included on this hat are 3.3V powered digital microphones which directly output I2S. This makes digital recording a snap with any microcontroller that has an I2S interface (and you can even hack it with SPI if you write the software carefully!). These aren’t audiophile-grade microphones, but for this application they are perfect. With current consumption of only 1.4mA and sensitivity of -26dBFS, they can record speech and ambient noises quite accurately. The application WizWorks designed these for was monitoring a data center. By streaming the audio and analyzing it, they were able to monitor the area for whatever sounds they were looking for.
WizWorks did an excellent job documenting the setup and usage of this hat with a Pi Zero and Raspbian! I think a Pi Zero with this hat, a lithium battery back and a 3D-printed enclosure could make an excellent speech recorder for doing audio interviews, or recording the sounds of nature on a hike. The limit is only your imagination. Check out the product page for more details!