Add ambient hum emitted from test chamber signage

[mwpenny]

What is the enhancement?

In the original Portal, start-of-level signs emit an ambient hum sound (sound/ambient/machines/fluorescent_hum_1.wav).

This issue tracks adding the sound to test chamber signs in Portal 64 once they have been activated. The sound should be emitted from the sign's location.

Additional context

For reference, see decor_object.c. Decor objects have the ability to emit sounds and the implementation will likely be similar for signage.