Unfortunately, it is copyright infringement unless the composer died over 90 years ago. However, another question is who is legally held responsible. Until a short while ago, at least in Germany the courts ruled that Youtube as the platform provider is responsible for the uploads and has to bear the consequences of copyright infringements. This is why the German copyright society GEMA has forced Youtube to ban certain videos (you often see a black screen with a declaration "blocked due to GEMA"). Youtube can block users whose videos lead to suits due to copyright infringement. But recently I read somewhere that theoretically a single user could also be held responsible by a court. So the legal situation with Youtube is not really clear.