Just like with the photos, there are many websites where you can find quality illustrations and vectors (usually SVG format), under public domain or Creative Commons Zero license, free for commercial use.
Finding a website that provides free music or sound effects for commercial use (under public domain or Creative Commons Zero license) is very, very hard, and there are only a handful of such websites right now.
Unlike with the photos or illustrations, there are not that many websites where you can find quality videos, licensed under public domain or Creative Commons Zero license, free for commercial use. You can find free videos all over the internet, …
If you need to find high-quality images to use on your website, but you can’t (or won’t) pay for them, the best solution is to get images from websites that offer free (for commercial use) images.