How Long Does Copyright Protection Last?
When an idea is expressed in a fixed medium, whether it be a painting, a story, a dance choreography, or a poem on the back of a napkin (though certainly not limited to those categories), it has legal copyright protections for a set period of time, allowing the creator to use or exploit the fruit of their work as they see fit, or not at all. In the past, this copyright required some formality in the way of registration or notice, but that is no longer the case for new works.
For more copyright information, read this complete Copyright LibGuide from Cornell University.
With printed music and recorded music, there are seperate copyright issues concerning the work itself and the item (score, CD, digital file, etc.). The work itself is one thing and will have its own creator and dates and rights, while the item (score edition, CD, etc.) will also have rights associated with it. For example, a Mozart symphony may be in the public domain, but the score edition published last year is not.
The simplest way to avoid music copyright infringement is by using music in the public domain, although the caveat remains: read the fine print on how to use material on any site. To record or use music in conjunction video does require additional licensing so keep that in mind.