Instead of Sine wave audio input, can I use dc coupling and give a dc voltage input to use the Class-D audio amplifier as a speed controller for DC motor (bi-directional)
Why not ..
After all, Class-D amplifier is just another version of PWM ..
For bi-directional control all of the above is true assuming that you supply the amplifier from +/- and 0V, motor is connected between output and 0V (half bridge configuration) and you don't have any dc input offset ..