This might go without saying, but I've seen schemes where frequency is adjusted to drive motors. The on pulse width time was not adjusted but the frequency was adjusted to speed up or slow down the arrival of the pulses. Of course, its the changing duty cycle the speeds up and slows down the motors, but one not familiar with PWM and/or driving DC motors might mistake this for the frequency controlling the motor speed.
In any case, a DC motor's speed is generally proportional to the voltage across its leads. Check the datasheet to see what the rated voltage of the motor is. If it is rated for 2.5 to 7v, I think common electrical knowledge (is there such a thing?) would alert someone to not drive it above 7v, but they might not worry about using 1.5v. In my experiences, this can also be bad. The lower voltage doesn't get the motor up to speed (especially under load) which can greatly increase the current through the motor, which will overheat it and cause premature failure.