Problem adding audio to video file

Hi, I am counting cars in a video and I am trying to make a sound each time I count one of the cars. In each video frame loop, I mark the cars with a 3x3 blue zone and add the frame to video, but it is not clear to me how to add the sound to the frame.

I am working with vision.VideoFileReader and vision.VideoFileWriter since those are the ones that allow me to make the video with audio.

I have read the help and googled a lot, but I still cant' find out how to add the audio.

If I go to the properties of the created video, it shows that there is no audio sample rate.

In addition, I can have the audio written to wav file, but I can't put it into the video.

I think the problem is related to the fact that the audio in the wav file is sampled, and the audio in my Matlab variable isn't...

Help, please. Thanks.