A microphone on Wio Terminal is certainly a mystery. Originally the hardware engineers told me the same thing, that it can only be used for measuring sound levels. I tried FFT demo and found that after adjusting the gain level to 7, the results were satisfactory (FFT displaying proper frequencies in response to sound played near Wio Terminal). So I started digging and put up a sketch for recording sound as .wav file to SD card with 8 kHZ sampling rate and 8 bit audio depth. That allowed me to create a beta version of Edge Impulse firmware with Built-in microphone enabled, which you can find here
and the video with article about creating Audio scene recognition
The audio quality will be improved in later versions of firmware. So far I have tested the recording with 16 kHz and 16 bit (rescaled from ADC maximum 12 bit) audio depth .
So, all in all, while microphone on Wio Terminal is not a professional recording microphone-grade (it does have a strong DC component, which can be partially rectified by using a filter), it is certainly suitable for audio recording/processing. Once I’m done with new Edge Impulse firmware (16 kHz 16 bit), I’ll publish the link here.