How to Quiet a Stairway?????

Does anyone have an idea on how to eliminate or quiet that hollow "thunk" sound you get when ascending or especially decending an un-carpeted stairway????
We are in currently refinishing our stairway, consisting of hardwood threads and pine risers. It's been carpeted since we moved in two years ago, and of course we never had the problem before because of that. However, ever since I tore up the carpet my wife's been asking me how we can eliminate that hollow wood sound.
I have full access to under the stairs, because there is a closet underneath.

Any ideas ????????????