First...
Code:

---

snd_tune (Handle, Volume, 0, 0);

---

And second...call the snd_loop from outside the while loop. Eg. :
You have:
Code:

---

while(1)
{
  snd_handle = snd_loop(sound_file_str,100,0);
  wait(1);
}

---

Change it to:
Code:

---

snd_handle = snd_loop(sound_file_str,100,0);
while(1)
{
  if(key_a == 1) { snd_var += 5 * time; }
  if(key_d == 1) { snd_var -= 5 * time; }
  snd_tune(snd_handle,snd_var,0,0);
  wait(1);
}

---

Hope this helps

EDIT: Right click > Add > Add sound > Sound file.wav
Right click while selected > Properties > Change the range (shouldnt be too small) and the volume. Build as normal (default) and enjoy stereo