Drive letters are easy. The problem is you wanted to remove any root folders as well. I do not know the name of these folders so cannot give exact code - I'm just assuming the root folder is named 'Music'.
How about this way -
$replace($directory_path(%path%),$left(%path%,2),,\Music\,,)\%filename%
$left(%path%,2),, - this removes drive letters.
\Music\,, - this removes folder named 'Music' (more exactly)
If you have other root folders, you can add to the code using that pattern. Like if you want to remove 'Film' folder, add -
\Film\,,
$replace($directory_path(%path%),$left(%path%,2),,\Music\,,\Film\,,)\%filename%
Just check the preview to see what happens.
(If this is no good, you'll just have to tell me the name of every drive + root folder you have. There is no easy code to do it automatically - it has to be specific to your folder structure)