What can I use to get the soot off my fireplace bricks ?

If the soot is inside your fireplace, then I wouldn't worry about it too much. Just use a brush. Maybe use water and a little soap. I wouldn't use muratic acid! Toxic stuff. The fumes will linger around inside your home for days, even if you don't smell it. If you go with the acid approach, then make sure that you open all windows.