I know "extracting song or artist names from Youtube" is not possible by using Youtube's data API. However, I have found several websites that have extracted the artist or song names successfully. I guess they are using machine learning or something else.
Is there any common practice to extracting those artist or song names?
You can extract artists and song titles from extra description, though there is one downside, the extra information may not appear in some countries (outside of United States), so you will need a proxy that is located in U.S, you can use Yahoo's YQL service to avoid this issue. The DIV element "watch-description-extra-info" will have all the extra information you need.