I will give some free tools to do that. It include download YouTube video and extract audio from video.
First download free tools. There are a serial of free video converters on the site http://www.etinysoft.com/free-video-converter.html. Here we use the iPad Video Converter. The following step-by-step tutorial will show you how to download YouTube video to iPad.
Click the drop-down menu button, select "Download Flash Video" item to launch "Total FLV Sniffer" http://www.etinysoft.com/images/free_video_converter/youtube_download.png
Play the web video in your IE(Internet Explorer) or Firefox, "Total FLV Sniffer will automatically detech and download the web video"
After download process completed, import it to "E.M. Free Video Converter for iPad" to convert the videos to iPad supportted video format.