Here is an introduction to synchronization rights, and what else you will need to use an audio track.
Copyright is a difficult issue in today's media market, especially when it comes to filmmaking. Much of filmmaking involves colliding separately created works, such as music or visual work, into one final product. Though you are creating a wholly new product with the aid of some of these pieces, you still do not always have the legal right to use them. When it comes to sound there are still some special rights that are different than video or still photo material. When you are trying to get the ability to use a certain bit of copyrighted sound you are looking to get synchronization rights.
Sync and Non-Sync Sound
Synchronization refers to the connection of the sound with the video. Traditional film stock does not record sound right from the camera and instead uses an external sound mixer to record the sound. This is called non-sync sound because the sound is recorded separately from the image. In post-production editing you then 'sync' the sound to the video so that it matches. With synchronization rights you are looking to go through a similar process. You are looking for the legal leeway to be able to synch a piece of copyrighted sound to your video image project.
Specific Sound and Copyright
The synchronization rights for your film allow for specific uses of the sound. The copyrights of the original piece of music or audio track is reserved by the creator of that track, and the copyright of your final film with that piece of synchronized sound is maintained by you. With this knowledge then the audio track is allowed to be used.
Really Using Outside Material
The synchronization rights do not necessarily give you final rights to use that track or to screen the film for financial return. To do this you will have to have a contract made between the distributor and the owner of the sound's copyright. To avoid these kind of issues it is better to get an overt agreement from the copyright holder giving you blanket permission to use that track for whatever purpose in regards to this specific project. It is a good idea to get this for each piece of external sound you get, and if you believe that there is no copyright held on the piece you are using then make sure that you accrue documentation of that as well. Public domain sound does not require these synchronization rights, or any other copyright agreement, which makes it prime for use. Creative Commons and Copyleft have a unique license that allows you to use if openly according to certain conditions, but you have to maintain that license on your final project. To get around this you will want to have those copyright holders agree to a synchronization rights deal that relinquishes the original license and allows a temporary blanket of freedom for your specific project.