Get Value Function Loop through all files in folder and its subfolders

I have a macro that uses the get value function to do a loop pulling the values in cell B15 through all workbooks in the folder "sample" on my desktop. This works great... Except now I need it to loop through all files that could be located in various subfolders of the "Sample" folder. But again it would have to loop through every subfolder of "sample" and then loop through every file in each sub folder.

I am having trouble altering the code to do this. I have attached my code below.