2017 Update: It's been awhile, I'm not updating this script anymore as you can see. I recommend you check out Arari's script
or a native browser extension.
Thanks to Magnus for his youtube2mp3, I though I'd change a few things so it wouldn't have to open a new tab every time.
All this does is just adds a button to next to the subscribe button that simply says "Mp3 Download" Enjoy!
Note: Haven't been here in a long time. Anyway, I have no problems with this script using Tampermonkey on Canary. But if you guys want a cool youtube player playlist thing you should check out 'Streamus' it's amazing and you should check it out. Check the reddit /r/streamus/ on how to get it setup because it was taken down from the Chrome store.
If you are having any issues with the button not showing up and whatnot, checkout jaszhix's version here https://greasyfork.org/en/scripts/11718-youtube-mp3-download-button-https
That's all guys, peace!