Artists own the rights to their music and it cannot be used without permission. Using music without permission will get it removed from Youtube and other streaming services, prevent it from being submitted to festivals, will not be allowed for class projects, and can even be seen as disrespectful to the artist.
Some musicians aren’t trying to make a living by selling their music and can upload it and tag it as “Creative Commons,” allowing anyone to use their music. The most common request is that they be credited, but some musicians will also set other limitations (like that it cannot be used in an advertisement). Because anyone can publish music under “Creative Commons,” the quality of these recordings varies from an eight-year-old playing with GarageBand for the first time to professional studio musicians that just want to share some of their compositions with the world.
When an artist passes, the rights to their music typically go to their estate. After a certain amount of time has passed, typically 50-70 years (depending on the country), the rights to their work lapses into Public Domain. Public Domain means that anyone can use (or record covers) of those songs. In other words, you can use classical music like Beethoven and Bach without getting permission from their descendants. There is no great resource to find popular Public Domain songs, but different organizations publish lists. As a rule of thumb, most things recorded in the 1940s or later are typically still controlled by the family estate. For instance, D.W. Griffith and Sergei Eisenstein both died in 1948, which means that their films can now be viewed and reused without requiring permission.
You can also pull sounds from Youtube and other streaming sites using a downloader like iyoutubetomp4.com, but know that the typically are licensed, copyrighted songs that you cannot legally use and are not acceptable in the context of this course. You can only use small clips that are sufficiently modified to make them your own and to warrant “fair use.” I would always recommend being overly careful when using popular music, which is why in shows, a character often only hears 5-10 seconds of a song on the radio and the producers are able to avoid paying royalties.
Fair use also applies to video clips, but it often lies in a grey area. For instance, if you used an explosion effect you found on a green screen in YouTube as a special effect in a student film, you would probably be protected. If Netflix used footage from a real life disaster in their commercial films, they would likely find themselves in trouble.