Re: motors are making a buzzing sound
Is your robot currently programmed so that it's being remotely controlled? If so, the problem may be that the joysticks are slightly off centered and sending a small amount of power to the motors, but not enough to make them move.
If you're using the older Radio Control Transmitter, you can adjust the "Trim" directly on the controller.
If you're using the newer VEXnet joysticks, you can recalibrate them: http://www.education.rec.ri.cmu.edu/products/teaching_robotc_cortex/index.html