The Notch 6 catalog pod cast has your answers. Basically:
Sound car: it can not be controlled. It has an on/off switch and manual sound control. The car does a variety of railroad sounds when the car is moving.
Sensor boxcar: you put in the engine TMCC number (not sure how) and when the boxcar crosses the sensor track a signal is sent to the engine to perform the pre-programmed function. Works with both Legacy and TMCC engines, so l assume the engine must be capable of performing the function.