Or 'Yes, Ma'am' if they have a female commander!

The answer can depend upon a person's status in the Army. Let's broaden the discussion to refer to "a military" such as Navy, Air Force, Marines, and "Army". Okay? The different forces have different traditions and they use different terminology. Any address of an officer is "Yes Sir." Sir is not used with any member who has the rank of "Non-Commissioned" officer. Thus, if I address a Sergeant or a Corporal, I do not have to use the term Sir. In the navy, the affirmation "Aye Aye, Sir" is used. ------------------------------------ In the Marine Corps, in Boot Camp training, Drill Instructors are Non-Commissioned Officers such as Gunnery Sergeant or Staff Sergeant are always addressed as Sir. Protocol requires that when an enlisted person walks toward an officer, a salute is offered, and held until the officer returns the salute. Then the hand is dropped. Enlisted personal called to appear before a superior officer, will stand at attention, an announce his appearance something like this. "Private Bain reporting as ordered Sir!" . http://en.wikipedia.org/wiki/Military_courtesy

There are many rules in the army.They answer "Yes Sir", I am not sure how to describe all the rules to you because there are so many.