Sound: Can be either a number or a filename (which must not begin with a digit).
- A negative number (e.g. sound = -1) will cause no sound to be played.
- A non-negative number n (e.g. sound = 5) refers to sound[n], as indexed in pak_dir/sound/sound.tab.
- A filename (e.g. sound = engine-start-diesel-0.wav) refers to a wave file located in pak_dir/sound/.