play_audio interface
- Definition：
int _stdcall play_audio(void* client);
- Parameter：
client:[in]，The client object which is form new_client.
- Return：
Return ERROR_OK if success，else refer to the definition of error number
- Explain：
Request audio data from camera.
Make sure the camera has connected now.
It will occur a AUDIO_STATUS_CHANGED event, usually,
the video status will follow the order-- STOPPED -> REQUESTING -> PLAYING.
When request successfully, the camera will send audio data to user by AUDIO_DATA event.