AV Pinout (PC Engine)
The PC Engine uses the same physical AV port as the Master System 1, Mega Drive / Genesis 1 and Neo Geo AES for the bottom 5 pins.
However, unlike all of the above, the PC Engine is wired differently, where it has Stereo audio instead of Mono, and the Right channel is on the pin used by the other consoles for Green video signal.
This means you need a specifically wired AV cable that wires the right audio correctly to get stereo output, and if you used a PC Engine AV cable on a Master System for example you would get audio noise as the green video signal would short to the audio output.
Here is the pinout looking into the console.