As the air is slowly removed from the space around the ringing alarm clock, the most accurate response is that the appearance of the clock will not change, but its sound will become fainter.
This is because sound requires a medium (like air) to travel through. As the air is removed, the medium becomes less dense, making it harder for sound waves to propagate. Therefore, the clock will sound quieter, but you will still be able to see it as long as there is still some light and visibility.