No it is illegal, with the following exceptions.
1. You are the author of the music
2. You are not the author but have an express license from the author
3. The copyright expired because the music and the recording is from before 1923
4. You are reviewing/commenting on the music itself.
Anything else it might be copyright infringement.
When you actually select the specific music, then you should consult a Copyright lawyer, because this site is for general questions, such as you posted and I have answered.
USPTO Registered Patent Attorney, Master of Intellectual Property law, MBA I am neither your attorney, nor my answers or comments in AVVO.com create an attorney-client relationship with you. You may accept or disregard my free advice in AVVO.com at your own risk. I am a Patent Attorney, admitted to the USPTO and to the Florida Bar.
If it's not music you yourself composed, and if you don't have a license to use it, using it as a soundtrack on youtube is copyright infringement, except if the music is in the public domain. It's not as simple as looking at whether the music was published before 1923, however. Even music first published before 1923 may not yet be in the public domain, as the recent NPR story about "Happy Birthday," which appears to date to the 1890s, suggests.
You may not distribute a copyrighted work without the permission of the owner.
Just because it is on Youtube does not mean that the person that uploaded the video does not have permission of the owner.
Copyrights expire. You can use music for which the term of copyright protection has expired without infringing on the authors rights.
A large portion of what's on You Tube infringes copyrights. It is not legal, but it is impossible for copyright owners to keep up with in many cases and the posters are usually individuals who cannot be economically sued. The material appears and then either the copyright owner finds the illegal posts enhance sales or finds they hurt sales. If they hurt sales, the copyright might decide to take action, which will likely be a DMCA take down notice to YouTube. All of this is unseen by the public as the posts proliferate so fast that they can hardly be taken down before another appears.
Copyrights are not self-policing. There are no copyright police. Someone has to take action to stop infringements and that costs money. Sometimes artists find the damage is less than the cost to stop it. However, the question to ask is "are you feelin' lucky today?" If you want to rely on the law instead of luck, you might want to see a copyright lawyer for a consult.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
No it is technically illegal. Note, however, that You Tube has entered in to settlement agreements with most major music publishers pursuant to which the publishers generally refrain from requiring You Tube to take down "tons of self-created" videos containing copyrighted music in exchange for annual payments of fees from You Tube to the music publishers. These deals keep You Tube out of trouble, but they won't stop individuals who post infringing content from getting sued.
This rarely matters. Copyright owners usually can't afford to bring federal copyright cases when someone post a You Tube video that only generates a few hundred "hits" or views. But I assure you that if someone posts a video with illegal copyrighted material that a million or more hits, the copyright owner will take notice and may sue the person who posted the video. In other words, the danger for individuals who post these videos occurs when they become popular, or when their videos go viral. As long as the individuals who post these videos remain in relative obscurity, they are rarely sued. This does not make it right to post copyrighted materials but it does explain why there are so many illegal videos on You Tube which exist with impunity.
If you're really interested in the subject of copyright in our digital world then read Lawrence Lessig's book "Free Culture: The Nature and Future of Creativity" [linked-to below].
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.