Part of what you're hearing on takeoff, especially with large, high bypass turbofans, is the sound of the airflow through the engine at low speed...not the jet effluence or jet noise, but the intake. As the airplane accelerates, the angle at which the incoming air meets the blades changes and the sound of the engine will change. The airplane will sound different when it's moving through the air quickly, as compared to the same power setting on takeoff.
The vantage point from which you listen to the engine makes a very big difference, too.