System command messages are interpreted directly by MCI. These messages do not rely on the ability of a device to respond to them.
MCI_BREAK
Sent by an application to set a break key for a specified device.
MCI_SOUND
Sent by an application to play sounds identified by entries in the [sounds] section of WIN.INI.
MCI_SYSINFO
Sent by an application to obtain system-related information about a device.