Experiments in vlogging with a headset & robot cam showed it's worth the investment in a more portable solution for recording audio. The need for VU meters, transport controls, & counters requires a wireless connection to a phone. Recording directly on a phone was horrible. The AGC brought up the background noise & clipped the voice. The phone & extra circuitry for monitoring was bulky.
The lion kingdom long dreamed of a wireless headset just to capture audio & monitor it. A small FM transmitter would be able to record the microphone but not send audio from the phone back to the headset. Despite every effort, there is no easy way to record from a bluetooth headset. The Goog has blocked recording from a protocol designed to make phone calls because of wiretapping laws. Commercially available, wireless headsets with full duplex audio are obscenely expensive, bulky, & have lousy sound quality. The only easily accessible ones have 1 speaker.
The most compact solution is a raspberry pi zero W reading from an ADC & writing data to an SD card. The same board sends monitoring output back to the headset. It's really hard to justify anything besides a raspberry pi zero W for every single project, nowadays. That board only uses 200mA & is the cheapest solution for everything.
A microcontroller is only necessary for converting the ADC's output to an SPI slave. An analog pot is the easiest way to adjust microphone gain. The ADC/microcontroller board would be dedicated just to the headset & interchangeable with a 4 channel version for less portable use. The 4 channel version would have digital pots for line level inputs, another section for monitoring, maybe mic preamps for 2 channels, lots of transistors for a patchbay.
It's important to use a CODEC or ADC specifically designed for recording audio rather than a bare ADC. Audio codecs have the lowpass filtering & DC offset correction you need to avoid aliases. What is the point, when a tiny soundcard for a raspberry pi is virtually free? It's about getting the best possible sound quality from the headset microphone. Past experiences with microphone inputs on computer soundcards were horrible.
The recording system could be implemented entirely in the headset if money was unlimited, but we're financially limited to an external board. This also allows the highest sound quality.