With Content Identification, a tool they use that scans new videos against a database of copyrighted content submitted using a form at http://www.youtube.com/t/content_management
Possibly. All music recorded in the United States is copyrighted unless it was a production by US government employees. Recordings produced prior to 1973 are covered under state laws and after that...
Unless the music is your own performance of your own original music, you would need a synchronization license for the song, and a master use license for the recording.
dirpy.com is good. If you have firefox you can also download the Easy Youtube Downloader add on from firefox.com, and download either entire videos or just the mp3 versions to go on your iPod or...
Audioswap or ask the record label to let you use it. You can also find songs marked 'Public domain' or 'Creative Commons' in which case you can use it for free. Commons requires you to credit the...