The adjective subdued refers to something, like a sound, that has a lowered intensity. Your loud conversation with a friend in the back of the classroom is likely to become subdued when the teacher passes out the exams.
The word subdued is related to the Latin word, subdere, which means "to subtract from." Something that is subdued has lost some of its strength or intensity. Subdued lighting might create a romantic mood at a restaurant. And a subdued child is one who is no longer having a fit. A subdued economy is probably pretty sluggish.