Hello, Joelsa.
Thanks for posting those very clear pictures and the video. Those current limits seem about right for that stepper motor, and the driver should be capable of handling them as well. I listened to the audio in your video carefully and I did not here anything abnormal at the end of the video. It seems like you might be hearing a normal high pitched whine from the motor caused by the way the current limiting on the stepper drivers works. Instead of using a current limiting resistor, current limiting on these drivers switches the power to the motor coils on and off very quickly like a switching regulator. The switching frequency is generally above the threshold for human hearing, but if you are young or have good hearing, it might still be audible.
The voltage on the motor leads you are noticing is also what I would expect. The current is required for the holding torque you mention and it runs through the coil, even when stationary with no torque. The motor you mentioned in your first post (#1205) is rated for 3.5V at a current limit of 670mA, so 3V seems about right with you running them a little bit under their current limit. If you want to turn off the current to the motors when you are done moving them, you can look at using the sleep or enable pins, which are discussed in the datasheet for the A4988.
I notice that you only have a single electrolytic capacitor in the corner of your breadboard for the motor power supply. You might try to place separate capacitors like that on the breadboard at the power inputs for the two stepper drivers. In general, this can help avoid LC voltage spikes, which could damage the board, and it’s possible that some inductance in the system is causing some harmonic that is making the typical switching whine slightly more audible.
-Nathan