how to close window

Hi, I wanted to search the forum and I know that there are threads about it - i read it myself sometimes ago - but I couldn't find it anymore. "window" will be ignored in the search ... So here my question. What is the best way to close a existing window in the outer wall, that it is maximum soundproof. Music style is metal, so I need isolation but the window must stay. I'm are not allowed to change the outside nor I'm allowed to make unremoveable changes e.g. close the window with bricks. So the idea was to fill the still existing window from the inside an put something behind. But what materials works best? What is the best construction. How heavy has it to be ...etc. I know there was an thread with exactly this problem with an solution sketch. But I couldn't find it :( Thomas