Or, for those who don't want to deal with the command line, a Google for "how to download music from youtube" will turn up numerous friendly programs to accomplish the task. Amusingly, there are a ton of YouTube videos explaining the process. :-)
On the producing side, it would also be easy to automate. Ffmpeg can add audio to a video, and it can trim a video. You could easily have a long video of your cat, say 30 minutes, and write a shell script to take a directory of MP3 files, and for each find the length, pick a random section from your long cat video of the appropriate length, extract a section of that length, and add the audio. The script could then upload the video to YouTube, putting the artist and song name in the description, so that on the receiving side naming and basic tagging of the MP3 could be automated.
