If they haven't left the rights to they descendants in their will, I believe that some rights expire after 70 years. So, if it's a modern author, most likely you won't be able to use music from them.
Anyway, YouTube has some kinds of agreement that allow you to use themes from some artists as long as you don't monetize the video, and let the music distributors place some advertisements in your video so that they can earn money through them. However, this also depends on the country: Japan and Germany have more strict monetization and copyright policies, whereas the South America and most European countries do not.