The general rationale for not using background music is that it increases harmful cognitive load. Cognitive load relates to mental processes (like perception, thinking, and organizing) used for thinking, learning, and working.
Working memory needs to process new information but it has considerable constraints (in capacity for new material and holding time). John Sweller, a well-known researcher and writer on memory and cognitive load and other aspects of learning, reminds us we must design with how our mental processes work. If we don’t, people can’t learn. And learning quickly is a mandate for current organizational conditions.