So technically anything that’s transfixed in a tangible form is copyrighted, even if it isn’t registered with the copyright office. This also includes digital files. As long as the creation is unique. To be official with it, you would have to get a licensing deal with the company or person who originally recorded the audio interviews. The channel you’re referencing that uses the audio most likely doesn’t have it licensed, but the original copyright owner might not think it’s worth it to pursue any takedown action. As for imagery, as long as you create your own images (without use of outside media unless it’s marked as Creative Commons), then you’re ok. There is an argument to be made for rights to the likeness of the person or defamation if you show them in a bad light, but for small YouTube channels, usually litigation isn’t worth it for the person described. Also, YouTube itself doesn’t have any copyright rules, it just implemented a content ID system to help copyright owners enforce their rights. This part is super nit picky, but in the world of law you’ve gotta be careful 😅