A schematic with the voltage in feeding into an Arduino's A?? port, and the MOSFETs or a driver chip being driven by the Arduino and into the stepper would be a good way to communicate the issue.
There's this example from Stepper.h:
This one from MobaTools:
...and this one from AccelStepper:
...but you'd need to change the loop to:
void loop()
{
// Read new position
int analog_in = analogRead(ANALOG_IN);
stepper.setSpeed(analog_in);
stepper.runSpeed();
// stepper.setSpeed(100);
// stepper.runSpeedToPosition();
}
This is all predicated on whether you can transform your voltage into a signal that analogRead() can read.