an audible sound from the motors is to be expected
the TB6560 stepper driver IC controls the motor current by switching the motor current on & off at about 1000 Hz on some boards
the frequency depends to some extent on the inductance of the motor
the 4 and 6 volts you measured is unlikely to be correct due to the complex shape of the AC voltage across the motor coils
once you have the motor data you may find the motor voltage is about 2 to 3V
most meters your likely to buy average the voltage being measured and are only calibrated to be correct for sinousoidal volatages like the 50 or 60 Hz mains supply
( not many are going £500 on a Fluke meter
https://instrotech.com/fluke-87v.htm...20group%20%231 )
did you find the part number for the steppermotors ?
unless you can down load the data sheet for the motors or the instructions for a cnc kit gives you the motor current you need to set the stepper driver to
you could either set the current too low so the motor torque is too low to move or
you set it too high and the motor over heats ( more than 70 to 80 C)
John