Dryer sheets are typically made from polyester, a synthetic fiber, which is not easily attacked or digested by bacteria or fungus (rot).
chances are it's not rotten dryer sheets, but rather rodent smells because the dryer sheets didn't work to keep them out.
like other replies have said, an ozone generator should take care of it. If no access to that, then you could try one of the 15 minute foggers by Meguiars. Most auto parts stores carry them.