Gravity zones and noises

[Altered Reality]

Whenever I set up a gravity zone with a nonstandard gravity value, I hear a weird noise (a low hum) coming from the gravity zone itself. You can hear it by running one of the tutorial maps about gravity. But I don't want the gravity zone to emit any noise, so I created an OGG file containing a second of silence (I named it silence.ogg), put it in the "sound" directory (inside "base") and inserted "silence" as the value for snd_gravity_loop.
However, when I load the map, I get a warning (WARNING: Couldn't load sound 'sound/silence.ogg' using default) and I get the same hum. What am I missing here? What must I do to have the gravity zone STFU?

I think this sound is in an .mtr file compared with the model by a shader.
If this is true, you have to replace the original sound path with your sound path.
After that, it could work but i'm not sure.
Maybe you make a copy from the existing file before you starting to replace the file
Chaos

[Altered Reality]

If I knew the name of the original audio file and where to find it in pak003.pk4, I'd just create the appropriate directories and slap my file there with the appropriate name, so the engine would load it from the actual directory instead of relying on the .pk4. Does anyone know the name of that file?

[Altered Reality]

I found the problem at last: the file I used was corrupt.
The default sound for the gravity zone loop is sound/entities/gravity_switch/gravity_switch_idle.wav : by creating the appropriate directories and pasting there a valid silence clip with the same name, I made the gravity zones silent.

Anyway, I have 2 more questions about gravity zones. Can they be activated and deactivated through a script? Can the gravity value of a gravity zone be changed through a script?