Is placing my website's name on a video or sermon that I did not record, or placing my website's link in a signature to attract visitors, considered theft or forbidden (haram), especially since the original owner of the material did not put their name on it?

Most audio and video clips of scholars on YouTube and elsewhere are not subject to copyright, and it is permissible to publish them to spread benefit, as long as they do not have explicit copyrights. If they do have copyrights, it is permissible to publish them provided there is no commercial use and no modification, and there is no harm in placing the website's name on them. It is preferable to separate transferred materials from self-produced ones.