The KM312-NT project implements the audio 3 mode of the ET312 or its clones such as the MK312.
It is a PCB of about 8x8cm that contains everything (including the power supply). One have to build as many as needed boards according to the desired number of channels.
It works with a supply around 10V. That can provided by 6 AA type batteries 1.5V or 8 NiMh AA type 1.5V or a single 9V battery.
It uses the PulsesGene project from ElecGeek user, here on Github. It adds the voltage to current converter and the transformer.