You would usually change the application or site that generates the links, and have that add the /sandbox to the URL.
If that's not possible, putting this into the web root directory (the one above /sandbox) should do:
RewriteEngine on
Options +FollowSymlinks
RewriteCond %{REQUEST_URI} !^/sandbox # If URL does not start with /sandbox...
RewriteRule (.*) /sandbox/$1 # Add /sandbox/ in front of it
(if it's easier to achieve with Alias, Apache Gurus, feel free to add your solution, but I couldn't get Alias to work for this scenario.)
However, this solution will make any other directory besides /sandbox inaccessible. You may want to re-think your Virtual hosts structure!