So you want to click the "Browse..." button on the upload box, find the filename, click "Open" and have that filename appear automatically in a box below? I assume the idea is that if the filename is incorrect for the mp3 (track01.mp3, rather than the desired Artistname - Trackname.mp3), the person uploading will be able to edit what the text is, but to cut down time it'll automatically appear?
If this is the case, it'll be playing with some Javascript, in which case try Googleing along those lines ('javascript update form field from another' etc.). The key will be getting it to strip everything but 'filename' from 'C:/path/filename.mp3'
If all you want to do is get the just the filename.mp3 to save into a database, then it'll be a simple use of the basename( $_FILES['upload_field_name']['name']) after submitting to extract the filename, and using that in the appropriate SQL query.
Hope this helps... it beats actually getting to work on redoing my site.