If airplanes can't use reverse thrust in flight, what am I hearing?
It is normal for engines to spool up during the approach. The initial part of the approach, from cruise level down to approximately 10 miles from the airport, is flown at flight idle power. This is the most efficient way to get down. In the final part of the approach, flaps and landing gear add so much drag that the engines need to be well above idle power. 

At low speeds required for landing, the wing of the aircraft will not produce enough lift to sustain flight. To compensate for the low speed, flaps are extended. They do not only increase the lift, they also increase the drag. When the flaps are extended, suddenly the drag increases, resulting in a deceleration. Extending the landing gear has a similar effect.

Normally the flight crew will limit the time that they fly at low speed and high drag. The first couple of degrees of flaps deployment are relatively efficient;they do not add a lot of drag but allow the aircraft to fly slower. When the aircraft is about 7 to 10 nautical miles from the runway, the crew will transition the aircraft to landing configuration for the final approach. Often there is a horizontal segment before the aircraft starts a 3 degree glide path down to the runway. The crew will ensure that enough flaps (drag) is added before they start this final segment, otherwise they have difficulty reducing speed (they usually try to prevent use of speed brakes in that part of the flight). 
Often there is a sequence of flap and gear extension before the final descent segment starts. This causes the aircraft to decelerate from the efficient high speed configuration (flaps 2 or less on the B737) to the landing configuration. At the end of that configuration sequence the aircraft has a lot of drag and is decelerating rapidly towards the final approach speed. To stop the deceleration extra power is commanded which spools up the engines and stops the deceleration. To reduce the spool up time of the engines, the engine control system (FADEC) will give a short burst of power. This what you hear while still decelerating. When the engines are spooled up they start to produce thrust which is the end of your deceleration. 

This youtube video shows how it happens. When at 28:20, the aircraft (Boeing 737) is flying horizontally, flaps 2, and is about to start decelerating and descending for landing. Keep a look at the thrust levers(right bottom corner of the screen) and the airspeed (on the left of the artificial horizon). 

It begins at 28:28 when the captain ask for flaps 5, and reduces the thrust in order to reduce to final approach speed. 15 seconds later he asks for gear down and flaps 15. The speed is now reducing rapidly. Starting at 28:50 the thrust levers are moved forward, until they are reduced again at 29:08. This is the power boost you hear. In 40 seconds the aircraft is transformed from low drag / low thrust to landing configuration.