At the women's center where I do volunteer work, our college interns seem to like to install YouTube downloaders on our computers. Many of the downloaders do nasty things to our computers, and can be impossible to remove.
If you want to put a downloader on your computer, Google the name of the program and check it out thoroughly. Don't just take the first downloader advertised on a popup while you're playing YouTube.

I record the sound from videos or whatnot on my computer with my Zoom Handy H2 recorder plugged into the earphone jack of my computer. Then I edit the sound files I've recorded in Audacity. Audacity is supposed to be able to record directly from the computer's sound card, but I was unable to do that after I got a computer with Windows Vista. With recent updates to Windows 10 and Audacity, I can once again record direct from my sound card - so I may not need an external recorder for that task any more.

-Joe-