Ok - here's how to do karaoke to your favourite video track, and publish the results on your blog
First, you need to get the video. Youtube will have it, guaranteed. There's a neat little linux package you can install to download youtube videos:
$ sudo apt-get install youtube-dl
Then go to youtube, and find the url. I chose 'people everyday' by arrested development - an oldy, but a classic, and its something I knew the words to. Then get it:
$ youtube-dl http://uk.youtube.com/watch?v=r9OjoPskf_c
It will download a 'flv' (flash) format video called the last part of the url - in this case 'r9OjoPskf_c.flv'
This next part isn't strictly necessary, but for ease of future access you may want to rename it, and also get it into some format understood by pretty much everything, e.g:
$ ffmpeg -i r9OjoPskf_c.flv people_everyday.avi
Then you need to record your soundtrack. I found the easiest way to do this was to use a linux tool called 'sound recorder'. Applications - Sound/Video - Sound Recorder. just open a new file and press record - simple!
Oh, but its a good idea to open your video exactly when you press 'record' - to get background and timings
Then when you finish press 'stop'. save it - e.g. 'people_everyday.mp3'
Then use mencoder to mux the mp3 and avi file together, like so:
$ mencoder people_everyday.avi -o output.avi -ovc copy -oac copy -audiofile people_everyday.mp3
Your output file 'output.avi' is ready to go onto your blog - like so!