Not a teacher
As far as I know, "sound" is usually meant to describe mental health when referring to a person. Saying "he is not sound" means that the person is crazy. "Not of sound mind" means that something kept them from making a wise decision (like being delirious with fever or suffering from dementia).
In the case of the "sound economy," that refers to something being stable and reliable. Someone can have "a sound plan" meaning that the plan they made is well thought out and likely to succeed. Someone can make a "sound choice," which means that they made a well thought out choice that is likely beneficial to the person who chose.
Hopefully that helps.
Student or Learner