I made an initial development board. This supports both sound-based and accelerometer-based ML tasks. As well as using the LEDs as a color detector. So this is intended to be used to develop and validate the tech stack. And then further cost-optimization will happen with later revisions.

These are the key components

• Microcontroller. Puya PY32F003
• BLE beacon transmitter
• Accelerometer. ST LIS3DH
• Microphones. Top-port or bottom-port MEMS, or external electret capsule
• Battery charger for LiPo/Li-Ion cells
• USB Type A connector for power/charge

Using a pre-built and FCC certified module for Bluetooth Low Energy, the Holtek BM7161.
This is a simple module based around the low cost BC7161 chip.

An initial batch of 10 boards have been ordered from JLCPCB.

Also did a check of the BOM costs. At 200 boards, the components except for passives cost

• With microphone:      0.66 USD per board
• With accelerometer:  0.825 USD per board

Additionally, there are around 20 capacitors, 1 small inductor, and 20 resistors needed.
This is estimated to be between 0.15 - 0.20 USD per board.
So it looks feasible to get below the 1 USD target BOM, for as low as 200 boards.

Also designed a small 3d-printed case, with holes for the microphone and LEDs / light sensor.