Is the use of clips from videos, movies, and songs in creating a special video, with modifications to avoid deletion and earn from advertisements, considered theft of others' effort? Is this permissible (halal)? And does earning profit negate the reward of good deeds if the original intention was for reward?

It is not considered theft or an infringement on intellectual property rights if one uses clips published on YouTube, so long as YouTube management has declared its usage policy to participants, because the participant is bound by the website's terms. It is stipulated for this to be free of religious impediment that the published materials be free of Sharia violations in content, images, and sounds, and that the earnings from advertisements be permissible (halal). If the original intention behind the work was to seek reward from Allah, then earning money does not nullify this reward.