You can download the video using online services or plugins for you browser. Then it should be either in mp4 or flv format.
To extract the audio, you can use command line tool ffmpeg (will also be able to convert to wav in the process).
The same can be used to convert the video into a format that Matlab will be happy with (could be just a collection of images).
Load the sound in Matlab (wavread), and low pass. Load the video in
Matlab (either by making a big matrix with the images, or by using the
VideoReader class). You can then probably also just low pass. I guess
this will blur any fast motion. For that reason you may want to convert
to a different color space first (e.g. rgb2hsl).
Produce either a video or a collection of images with Matlab.
Then recombine audio and video using ffmpeg.
You should find the command lines for ffmpeg by Googling around.
Hope that helps,