When creating my website is a crime to copy other people's photo, audio or video via URL and uploading it online on my webpage?

Answer 1

no if the owner of that resource allow it, and you fulfill the criteria that they set so you and other people can use it.
usually in form of not use it as comercial purpose, or give an attribution or link back, some are generous enough and you can use them freely
sometimes though you need to buy it. not just "buy" the work, but buy the permission to use their work. for example in audio video media like song, by buying the song or album doesnt mean that you're allowed to use it in your website/broadcating it. what you buy is the permission for your personal entertainment.
if they dont say anything about the use of their work (music, photography, etc) better play it safe and dont use it. or you can ask

Answer 2

You must first seek permission from the owner or give credit for what you use