Locate the first missing file in the list by highlighting the name of the file (just one file name) in the dialog, clicking locate, and double clicking an exact match of the same file name in the provided dialog. You'll notice that when you return to the "missing sound files" list, that file you just located is no longer in italics, but the other files are.
If those other "missing" files are located in the same folder (as they should be), you should be able to click one file name at a time, and simply click "locate" without having to go through the dialog again... what you should see as you do this is that file names in the list are no longer italicized, and their locations are listed next to their names.
Once ALL files in the "missing sound files" dialog are no longer italicized, proceed by clicking OK. It is important that you make sure that ALL listed files in the dialog are located before proceeding.