(svn r2685) -Codechange: Split the music/sound/video drivers into separate files and move them into subfolders.
This results in shorter and hopefully easier to maintain files.
Note: I had to change paths in #include statements of some unrelated files, because I added the ottd base directory to the include path (-I.)

#ifndef SOUND_NULL_H
#define SOUND_NULL_H

#include "hal.h"

extern const HalSoundDriver _null_sound_driver;

#endif