I am not sure if this will help, but if I leave towels in the washer overnight and they develop that "musty" smell I rinse them with 1/2 cup white vinegar added to the rinse cycle. Maybe you could take the cushion covers off and wash on gentle and spritz the foam with a vinegar and water solution?

Megan

Service Master seems to have some sort of product that really removes musty as well as smokey odors.
I would call them.
Linda C

Here is a link that might be useful: Service Master

I have very bad allergies to musty smells. I find spraying with lysol helps me better than anything else I've tried.
ivamae

Try sprinkling Borax under the cushions and letting it set.

Try taking the cushions off, put them in a strong plastic bag, wrap the bag around the nozzle of the vacuum and turn it on. The idea is to take out all the air from the cushions, and it should help with removing the smell. When you see the bag flatten out, turn the vacuum off, take the nozzle out, and let bag refill with air. You can also try tucking a few unused dryer sheets into the cushions, and around the arms. Also, try vacuuming the arms and back with the crevice tool. It creates the most suction and should help to remove more of the odor. Good Luck!

If mold or mildew is growing inside the sofa, it's time to part company with it.