Not exactly sure what you want to do so I'll put what I think to check:
You want to use the audio from an existing video file as the soundtrack to a completely new video you are making which is made of of a picture slideshow?
If that's wrong you'll need to correct me obviously!
I would use windows movie maker, I just made a picture slideshow of my mates in funny photos with the Danger Zone as the soundtrack. It was ridiculously easy. All you have to do is drag the photos onto it, drag the music onto it, choose which transition animation you like the look of and your done. Of course you can edit the lenght each photo is on for etc to match the music and you can also trim the audio if you need to as well.
The only sticking point you have now is... you need to extract the audio from the video. Possible, I'm not sure how to handle youtubes .flv files, I'd try this site I found
http://vixy.net/ (not used this site at all, just came up on google) to convert it to .avi and you should be able to drag the video to the audio level on the timeline in movie maker to use just the audio and not the video.
You may or may not have windows movie maker already on your PC, but version 2.1 is free to download if you have a genuine copy of windows.
Good luck.