I just finished a musical quilt for my DGD. I had a hard time finding musical stuff. You just have to keep looking in every LQS and chain stores. Also ebay. To give a musical feel I made notes (machine appliqued) and # to solid colored fabric. Each note was in it's own block. For DGS I appliqued a drum and used it as a center for his quilt. I have seen colorful guitars out there also.
Good luck. Half the fun of making a theme quilt is fabric hunting. It took me 9 months to find enough varity of music stuff