For Role Scoper's rewrite rule to handle the URL, the folder must be the WordPress uploads directory, or a subfolder of it.
For Role Scoper to enforce permission based on some page, you must make that association by storing the audio file as an attachment. However, that attachment does not need to be visibly linked. If the WP upload process is too cumbersome, you could make it work by manually executing the following query, once for each audio file:
INSERT INTO `wp_posts`
( `post_parent`, `post_author`, `post_category`,
`post_title`, `post_name`, `guid`,
`post_date`, `post_date_gmt`, `post_modified`, `post_modified_gmt`,
`post_status`, `post_type`, `post_mime_type`,
`post_content`, `post_content_filtered`, `post_excerpt`, `to_ping`, `pinged`)
VALUES
(PageIDhere, 1, 0,
'filename', 'filename', 'http://yourdomain.com/wp_folder/wp-content/uploads/your_subfolder/filename.mp3′,
'2008-12-30 22:21:17′, '2008-12-31 02:21:17′, '2008-12-20 22:21:17′, '2008-12-21 02:21:17′,
'inherit', 'attachment', 'image/audio',
", ", ", ", ");
I arranged the column order to make this as logically convenient as possible.
You need to determine the ID of the page which will determine permissions and put that in place of PageIDhere. Optionally, set the date, author and/or category. Then run the same query once for each audio file, changing only the filename.