If you need background music check out Bensound, which provides royalty free tracks.
Sample MLA Citation:
Tissot, Benjamin. "Acoustic Breeze." Bensound, 2018, www.bensound.com/royalty-free-music/track/acoustic-breeze. Accessed 1 Nov. 2018.
Why am I citing this soundtrack? How do I know I am eligible to re-use it?
These soundtracks are made available for re-use under a Creative Common's license which requires that you provide credit/attribution. The MLA citation gives credit and allows your viewer to locate the original source.
If you are using soundtracks provided within a webtool such as wevideo or iMovie you do not need to cite the music.