I have several Featherweight cases that smelled rather badly. The first thing I did was wash the inside with soap and water and dry thoroughly. I put coffee beans in the box and closed it. When summer comes and the boxes still have the musty odor, I put baking soda in a container and put that into box and close it up. Then I put it in my car on very hot summer days. If cleaning it with soap and water and putting the coffee beans in them didn't work, then the heat of the car does the job.
What causes the smell is mold and the heat kills the mold and destroys the smell.