This is an indirectly art-related topic: if you’re a sampler and mixer like myself, internet videos are a treasure trove for samples. That said, the ability to pull the sound from the video and work it into a song, mix, jam, or use it as a skit, is valuable.
Unfortunately, this is one issue where everyone and their dog has their own goddamn idea about how this should happen. Download this app! Sign up for a subscription to this program! Use iTunes! Use ffmpeg! Use VLC! Ableton has this capability natively!
To all of these I say, “Nay.”
There are two painfully easy methods that, as far as I looked, have been ignored by the internet community for too long. Here they are.
1) With your downloaded video, whether its format is .mp4, .flv, or .avi, simply select the title as if you were going to rename the file. Then choose the format ending and change it to .mp3. The operating system will ask if you want to keep the original, or use the new ending.
2) Use Audio Hijack. Personally, I like to rely on as few apps as possible and minimize my downloading, but this program is awesome. I’m not sure of a way to get the Pro version for free, but the trial has worked fine for what I’ve needed to do.
That’s it, kids! Don’t waste your time on Howto following 500 steps only to find on step 458 that the instructions refer to iTunes 6, while you’re rockin’ iTunes 11 (that just happened to me).