If you would like a complete unit on math in music, check out this comprehensive module complete with lesson plans, materials, and instructions on teaching math concepts through the study of sound (ex: frequency, hertz, etc)
Using the theme of musical instruments in a 100-piece orchestra, these activities use real-world situations to teach topics like combinations, permutations, word problems, sets, ratios, and fractions. (Though the lessons are listed as for elementary, many of the options listed are inclusive of 6th-grade math classes.)
Teach concepts of ratios and more through basic jazz concepts and octaves in this series of complete lesson plans. These 8 lesson plans explore learning ratios and measurement via music notation (i.e. creating blues progressions).
This lesson plan is a great way to include hip-hop in your classroom in a way that conveys a respect for hip-hop as an intelligent art form. This lesson includes a video and activity of hip-hop producers discussing how they use math in producing music and setting up a realistic hip-hop production algebra challenge for students. It also includes additional interactive algebra challenges to extend the lesson with further practice.
Great for non-musicians who would love to put music into their math but are not exactly sure where or how to do so. This 5-part lesson breaks down the various places in music you can find math concepts.