In this research we analyze the influence of musical activities in the acquisition of mathematical knowledge and skills of a sample, 50 students from both a public and a private school in A Coruña (Spain), at a cognitive level. Based on a quantitative study with a quasi-experimental design, we evaluated students’ knowledge acquisition; we worked with musical activities related to mathematics in the experimental group (EG), and with traditional mathematical activities in the control group (CG). We used a questionnaire that the teachers completed before and after putting the activities into practice, after collecting—writing daily field notes—the mathematical knowledge acquired by the students. The results indicate that there are significant differences between the pretest and the posttest, between CG and EG, but there are no differences between public and concerted schools. In short, it is concluded that music represents an excellent tool in mathematical learning.