Everyone uses music in their highlight reels and edited video productions.
Wedding or event clients may request a specific song - the client may even give you a legally purchased and downloaded copy of the song to use. But it is important to understand that purchasing and downloading a song for personal use does not give you the right to use it in an audiovisual production - especially a video that you produce and sell for money.
A copyright is at its heart much the same as any other property.
Owning the copyright means you get to exclude others from using it, and a copyright owner typically exercises this right by only selling the right to use the copyright in specific ways, and thus excluding every other potential use. Thus, if you purchase and download a song from iTunes, you have only purchased the right to listen to that song yourself or with a small group of family or friends. You cannot perform or broadcast that song publicly, at a bar or venue or over the radio or internet. You cannot make another copy of that song (aside from incidental copying for personal use). You cannot alter or edit that song. And most importantly, you cannot include that song in a new audiovisual work. In order to do any of these things, you must purchase those rights from the rights holder. Doing so is usually not difficult, as most music rights are held by large companies that sell licensure for standard rates - the biggest and oldest music licensing companies are ASCAP and BMI, but TuneCore, Rumblefish, and YouLicense are all new players on the scene.
Failing to acquire the proper rights, however, can be a hassle. Legitimate rights holders - as well as copyright trolls - can and do sue photographers and videographers over improperly licensed songs included in highlight reels and videos, and because most rights holders are well-organized corporations with a thorough system of documentation, these cases can be very costly and hard to defend.
There are two ways to protect yourself from this kind of lawsuit.
Ensure that you always properly license the music that you use. Talk to a copyright attorney if you are uncomfortable navigating a music licensing company, but the principle is simple; just make sure that you are purchasing the right to use the song in an audiovisual work.
When a client brings you a song to use and claims to have the rights themselves - maybe they wrote the song themselves, or it’s a recording of their child performing a song. Your contract should include an indemnification clause that states that in the event that you are sued for copyright infringement on the basis of any song provided by the client, the client will cover the costs of your defense and any judgments against you. Be careful here! This indemnification will not cover you if you negligently fail to license a song - it will only work if your client provides music they claim to have the rights to use. Always double check whether rights are available for purchase before agreeing to use an unknown song in a video product.
It’s also important to note that a performance of a copyrighted song - a singer performing with a band, for example - requires its own licensure. If you are taping an event that includes live performances, you will need to license the songs performed, or at the very least require that the client warrant that they have the proper rights to the song and will indemnify you in the event of a lawsuit. Don’t hesitate to speak to an attorney to help you sort out the tricky bits of music licensure to make sure you don’t get sued for copyright infringement.