The ACMDRV_MESSAGE_PARAMS structure is for parameters passed to the ACM_IOControl function.
typedef struct { DWORD dwDriverID; HANDLE hDriver; UINT uMsg; LPARAM lParam1; LPARAM lParam2; } ACMDRV_MESSAGE_PARAMS;
The ACMDRV_MESSAGE_PARAMS structure is declared in the header file Wavedev.h.
ACM Driver Messages (ACMDM_XXX)