this is my code
Code:
#include <acknex.h>
#include "default.c"

var volume = 0;

SOUND* test1_wav = "CI_Battlefield.wav";
SOUND* test2_wav = "Ci_Intro.wav";
SOUND* test3_wav = "Sword01.wav";
BMAP* test1_pcx = "option_volume.pcx";
BMAP* arrow_pcx = "arrow.pcx";

function main()
{
    video_mode = 7;
    screen_color.blue = 150;
    mouse_map = arrow_pcx;
    mouse_mode = 2;
    while (1)
     {
         mouse_pos.x = mouse_cursor.x;
         mouse_pos.y = mouse_cursor.y;
         wait (1);
     }
}

function volume()
{
	// what code in here?...
}

PANEL* main_pan =
{
    bmap = test1_pcx;
    pos_x = 0;
    pos_y = 0;
    hslider (35, 182, 349, "slider.pcx", 0, 100, volume);
    digits (15, 50, 3, _a4font, 1, volume);
    flags = VISIBLE;
}