// coresound.h

#include <allegro.h>
#include <logg.h>

#define SP_NORMAL 0
#define SP_ONEVOICE 1

#define NUMVOICES 256

struct obj_t;

typedef struct
{
	obj_t* object; // ERROR: syntax error before "obj_t" 
	int playmode;
} voice_t;

extern voice_t* voicelist[NUMVOICES];

extern LOGG_Stream *oggsong;

// object.h

...

#include <allegro.h>

#include "r_actors.h"

...

typedef struct
{       
	unsigned short int sprite; // Index of sprite to be drawn at object's position
	unsigned char drawstyle; // Manner in which to draw the sprite
	int x; // X Position
	int y; // Y Position
	int momx; // X momentum
	int momy; // Y momentum
	int type; // Dictates object's identity and behavior
	...
}obj_t;

...

extern obj_t objectlist[MAXOBJS];

...