The statuc wouldn’t have to be loud. As long as you can make the player notice the speakers that the voice is announced from at the beginning of the game, by the end of the game they should be able to notice the speakers as well as noticing that the quiet static coming from it signifies that the military has abandoned the base.
It’s been a while since I’ve played Half-Life, so I can’t remember if it’s actually silent at all. I just based that off what UrBY said. Regardless of whether or not it does go silent, I still think that the silence makes sense. The military took over control of the voice announcement system, but by the ‘Forget about Freeman’ chapter, they definitely seemed to have lost control of the situation and that would include the announcement system. It would be a very powerful feeling to have the announcers go silent as a way of signifying that no one is there, and reinforce the notion that the last hope of humanity has abandoned the facility.
How would you know that? Aside from being a very flawed idea announcing everything from the citadel (sound can get lost and distorted very easily for one), it would be a lot of hassle. I always thought they sounded as though they were coming from speakers dotted around the city like the Breencasts. I always got the feeling that they were about 30 meters away.