I'm not sure if I’m going to suggest a different approach, or if I’m going to tell you in different words what you already had in mind with your original question, but since you want the nodes to be files, I would rather generate the nodes starting from the files, rather than linking existing nodes to existing files.
In general terms, I would do this programmatically: for every existing file in your import directory, I would build an object $nodeand then run node_save($node)it to save it to Drupal.
Of course, when creating the object, $nodeyou will need to call the API function of the module that you use to manage the files. Here is an example of the code that I wrote to perform a similar task. In this case, I attached the product sheet to the product (a node with additional fields), so ...
field_sheet CCK nodeproduct node$sheet_file - ( + ) .
, :
$field = content_fields('field_sheet', 'product');
$validators = array_merge(filefield_widget_upload_validators($field));
$files_path = filefield_widget_file_path($field);
$file = field_file_save_file($sheet_file, $validators, $files_path);
$node->field_scheda = array(0 => $file);
unlink($sheet_file);
BTW: MP3, $ node → .
, !