How can I record the sound off a youtube video and make it an audio file?

There's a couple different ways to do this. One is to have a microphone on and place it near a speaker and hit play on the YouTube file. The other is to just get your settings right and play the video and have it record. All this hinges on finding an audio recording program, you can find a lot of free ones (and some not-so-free free ones as well, if you know what I mean ;). They're quite easy to use [the basic ones anyway].

each video has two tracks the video and the audio you can use pinnacle to strip the audio off the video to use it

You can use a software by searching easy record at download.com which allows you to record directly from the sound card.