What is the effect of background music in educational videos on the learning outcome?
See Moreno, Roxana, and Richard E. Mayer. "A coherence effect in multimedia learning: The case for minimizing irrelevant sounds in the design of multimedia instructional messages." Journal of Educational psychology 92.1 (2000): 117. http://dx.doi.org/10.1037/0022-0663.92.1.117


  The authors tested the recommendation that adding bells and whistles (in the form of background music and/or sounds) would improve the quality of a multimedia instructional message. In 2 studies, students received an animation and concurrent narration intended to explain the formation of lightning (Experiment 1) or the operation of hydraulic braking systems (Experiment 2). For some students, the authors added background music (Group NM), sounds (Group NS), both (Group NSM), or neither (Group N). On tests of retention and transfer, Group NSM performed worse than Group N; groups receiving music performed worse than groups not receiving music; and groups receiving sounds performed worse (only in Experiment 2) than groups not receiving sounds. Results were consistent with the idea that auditory adjuncts can overload the learner's auditory working memory, as predicted by a cognitive theory of multimedia learning.