A auxiliary audio device driver must have an entry-point function named auxMessage to process messages sent by MMSYSTEM. The syntax of auxMessage is:
DWORD auxMessage(uDeviceID, uMsg, dwUser, dwParam1, dwParam2)
The uDeviceID parameter is a UINT specifying an ID for the target device. Device IDs are sequential, starting at zero and ending at a value equal to one less than the number of devices the driver supports.
The uMsg parameter is a UINT specifying the message being sent to the driver. The driver should return MMSYSERR_NOTSUPPORTED if it does not support the specified message.
The dwUser parameter is not used.
The dwParam1 and dwParam2 parameters are message-dependent.
The low-order word of the return value specifies a message-dependent return value or error code. The high-order word should be zero.