The MMSYSTEM.H header file defines data types and function prototypes for all of the audio functions. You must include this header file in any source module that uses these functions. MMSYSTEM.H defines the following data types for the waveform output functions.
| Type | Description |
| HWAVEOUT | A handle to an open waveform output device. |
| WAVEOUTCAPS | A data structure used to inquire about the capabilities of a particular waveform output device. |
| WAVEFORMAT | A data structure that specifies the data formats supported by a particular waveform output device. This data structure is also used for waveform input devices. |
| WAVEHDR | A data structure that is a header for a block of waveform output data. This data structure is also used for waveform input devices. |