Is it possible to get a refund, credit, or rebate on the price of an
airline ticket if the specific flight drops in price from the time of
purchase and the time of travel?  (this is a general question applying
to any airline, any segment, or any ticket type)

Since each airline has its own policies and each ticket has its own
restrictions, there is no way to get a general answer for all airline
tickets. In my experience, what they the large domestic US airlines
have you do is cancel your first ticket and then buy a new one at the
lower cost.  If your particular ticket is nonrefundable, then you will
have to pay the change ticket fee (if the airline allows on this
particular ticket per the rules of the fare). So, unless you have a
fully refundable ticket, or unless the fare has dropped more than the
change ticket fee (typically $50-$100 per ticket), then you will not
be able to pocket any of the savings.