MCPX

The MCPX is the southbridge chip of the Xbox chipset by Nvidia. It contains the sound processors (APU and ACI) and also the USB, PCI, IDE, etc, controllers[1], [2].

ROM

The MCPX is home to the secret MCPX ROM.

Pin L21: PC Speaker

The MCPX has PC Speaker pin which can be controlled using [the standard PC Speaker interface https://wiki.osdev.org/PC_Speaker]. However, no actual speaker is connected to the pin, so while the signal exists, there will be no audible sound on a stock Xbox.

A speaker can be soldered to this pin and to make the signal audible [3][4]

The original Microsoft code does not drive the PC Speaker at all, so this otherwise unused pin can also be used for inaudible forms of unidirectional communication.