How do I add audio to a .avi file?

I have a .avi file I made from a series of image frames using VideoWriter. I want to read it back into Matlab and add some audio to the video. What is the best way to do this? (I can't find a clear answer online). Furthermore, using VideoWriter, I changed the frame rate so the images would play slowly in the .avi. (The data that they come from has a sample rate of 100 samples/second, but the video is playing at a rate of 20, so 1/5 of the speed. Is there a way to, when the audio is added, to slow down the audio so it matches the length of the video? (I want to be able to automate this, so that it will change automatically based on the frame rate I input to Matlab).

Thanks!