I took a deep look into the WiFi hardware on the Raspberry Pi Zero W.

The chip used is the BCM43438 (which actually seems to be designed by
Cypress Semiconductor), and also the same one used on the Pi3 Model B
https://www.alldatasheet.com/datasheet-pdf/pdf/1018493/CYPRESS/BCM43438.html

First, it's not USB connected as I'm sure I've heard suggested
before. The chip actually supports either SPI or SDIO communications
and the description of the sdio overlay in /boot/overlays/README
suggests that the Pi defaults to communicating with it using SDIO
in the 50MHz "high-speed 4-bit mode" (200Mbps maximum speed quoted
in the datasheet). SDIO is the protocol used by SD cards for
high-speed data transfer.

It also has a built-in broadcast FM radio receiver, controlled via
the same UART interface as the Bluetooth controller. This is able to
output analogue stereo audio, or provide a digital stream. Most
likely the pin that the FM antenna needs to be connected to isn't
connected on the Pi circuit board, though you never know maybe it
goes to one of the undocumented test pads?

Not mentioned on this list for the Pi3B (though some entries are
blank):
https://raspberrypi.stackexchange.com/questions/91901/what-are-the-functions-of
-the-test-pads-on-the-pi-3b-3b

I don't know of a comprehensive list for the Pi Zero W's test pads.
The schematics are useless, they don't even show the WiFi chip!

The commands for controlling it over the UART connection are probably
documented in Cypress' WICED SDK package.
https://community.cypress.com/message/179340

It would certinly be fun to combine this with the well known GPIO FM
transmitter hack, to make a Raspberry Pi walkie talkie. But I'm
guessing that the required pin/s are probably stuck under the
chip (curse these modern chip packages! :) ).

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