I do not think that you will be able to play just the sound from a Youtube video. You would have to play the video as a whole. You could potentially add a subview above the Youtube UIWebView. Here is a link to check out that deals with the Youtube API:
http://apiblog.youtube.com/2009/02/youtube-apis-iphone-cool-mobile-apps.html Honestly, don't try and get into stuff like this if you just finished doing a single Hello World app.
What you would have to do is make an IBOutlet for the UIWebView, put the UIWebView in a view in interface builder, then go and link up a default URL to the UIWebView. Automatically running a Youtube video is very hard because the iPhone does not contain flash so you would have to run the Youtube media player. This does this by default when you click a link in a UIWebView that contains a Youtube video but I do not think that it can just start playing a video without any other sort of additions.
