Int _stdcall start_speak(void* client, int enabled);
client:[in]，The client object which is form new_client.
buffer_ms:[in]，The buffer size of camera which is used for play audio, millisecond
Return ERROR_OK if success，else refer to the definition of error number
Request to play the audio on the camera which is captured in client terminal.
Make sure the camera has connected now.
It will occur a SPEAK_STATUS_CHANGED event, usually,
the video status will follow the order-- STOPPED -> REQUESTING -> PLAYING.