Just because a file is available online does not mean it can be redistributed or modified.
Many sites offer files and media that are covered by Creative Commons licenses that permit use or the creation of derivative works.
The sites listed on this page either offer all their files under a Creative Commons license or make it easy to limit searches or results to Creative Commons licensed materials.
None of the sites requires a sign-up/log-in to search or download materials.
Concerned that creators might change their mind about the type of Creative Commons license they assign to their work?
Consider using Archive.is or another webpage capture tool to document the copyright status of work that you find online.