I have Audacity version 2.0.5 and my computer runs on 64 bit Windows 7. When recording from YouTube to iTunes I only get about half of the playback volume. My english is not very good, and I am probably misunderstand something in the Help topics.
Turn both the sliders up in Mixer Toolbar:
In the web browser, turn the volume slider up on the Flash player.
Also turn up the volume of the flash player in Windows. Left-click the speaker icon by the system clock, then click the “Mixer” link, then look for the slider with the Flash player logo.
You may find these tutorials from the Manual useful: