In the stock firmware there is the following code in the '/etc/init.d/rcS' file:
#power on sensor /bin/gpio_ms1 -n 6 -m 1 -v 1
'gpio_ms1' binary when compiled with '-DSN9866X' flag maps gpio_ms1…gpio_ms1 on aud…aud. In other words for each call with '-n X' flag where X is a number from 3 to 7 it internally calls 'gpio_aud' binary with argument 'Y' where Y is X minus 3. So that the line above means that aud is set to high.
aud line of the SOC is going to pin24 on the camera board connector. The line from the pin goes to the seat for a resistor which is left unsoldered. There are may be other vias under the socket however but the fact that even without setting aud high the sensor works well.
The speaker is driven by the SGM4895 power amp. The shutdown controlling line “nSHDN” of the speaker is connected to aud line of the SoC. So that you need to set aud gpio to high in order to play sound.
The 3V3 bus of the wifi module is sourced from the p-channel MOSFET WP3401 (sot23, mark WP1J). The gate of the transistor is pulled down by a resistor and connected to the line gpio of the SoC. If you need to hard-reset the wifi module then you should kill all networking software, unload the driver and set gpio to high for a few seconds.