In my humble opinion, I would not use anything that someone else made, unless you have expressed consent from them (i.e., a letter, an email, etc.) If you acknowledge that person x made x thing in the video, that might not be enough. For example, I made the mistake of putting a 30 second The Beatles music clip in one of my videos and I got a notice. I had put in the description that it was x song by The Beatles but that wasn't enough, even when my video wasn't monetized.
I am not trying to scare you, but I would just try to keep things as safe as possible. You could use royalty-free music, but make sure the author says that it is okay to use in youtube videos. They usually ask you to give them credit, so look for that.
I hope this helps you out! :)