Instrumental music is protected by copyright laws. If you wish to use instrumental music that has been published and copyrighted, you may have to purchase a licence from both the copyright owner and the publisher. There are various types of license depending on how you wish to use the instrumental, for example, for commercial use, for performing, on CDs and records or for digital configurations such as full online downloads, limited-use downloads, on-demand music and video streaming.
TL;DR (Too Long; Didn't Read)
Generally, you'll need to purchase a copyright license to use an instrumental legally. The only exceptions are where you're using an extract of the music for education purposes or the instrumental is so old it has fallen into the public domain.
Check the Copyright
First, determine if the instrumental that you wish to use is public domain, freeware or copyrighted music. If you have a copy of the sheet music, examine it for a copyright notice on the bottom or back page. Works that are public domain are any copyrighted materials that were published on or before Dec. 31, 1922, or it has been 70 years or more since the date the composer died. This includes any instrumentals that were never published but were composed during the musician's lifetime.
Consider the Fair Use Exemption
Determine if your use of the instrumental is covered by the Educational Fair Use Guidelines. Fair Use includes using the music for learning or educational purposes such as performance of and displays of the music in face-to-face teaching and distance learning and its inclusion in educational course-packs. You can use the instrumental legally for educational purposes as long as you possess a legal copy of the work. You must include a copyright notice with appropriate citations and attributions to the source of the music when you use it. Note that if you are going to use the instrumental for commercial uses or to make money, it is not legal to use the fair use exemption. You'll need to purchase a licence first.
Find the Copyright Holder
Find out who the copyright holder or publisher of the music is if the music is still under copyright If the publisher or copyright holder is listed on the music, the address of the may already be printed on it. If it is not, use the Music Publishers Association's Music Publisher Directory or the Index of Publisher's Imprints to find their contact information. If you cannot locate the publisher on the mpa.org website lists or you do not have a copy of the music with the copyright holder's name on it and have no idea where to start to look for the copyright owner, visit one of the following organizations: the American Society of Composers, Authors and Publishers, Broadcast Music Inc. and SESAC Inc.
Fill Out a Permission Form
Search the Music Publishers Association website for "Copyright Search Permission Forms." Download and print the form that you need. The three types of forms are for using out-of-print music, arrangement permission and photocopying or getting out-of-print music. Fill out the form that most closely fits the description of what you wish to do with the instrumental and send it to the copyright owner or publisher and wait for a response. They will either tell you that you need to pay for a licence or they may deny you permission for use and you will have to find another instrumental.
Find Free Use Instrumental Pieces
Look for instrumentals on the Internet that artists have put up for free use on websites like freemusicpublicdomain.com. There are many artists that willingly create music that is intended for free use and are available for download. If you wish to use music on a video that you want to upload onto a social networking site or a video sharing site, you can use AudioSwap, which was created by YouTube, to legally add music from their list to your video. AudioSwap provides a list of songs that they have licensed for you to use.