Thanks for the reply. All the files are served from the root directory. Even using a hardcoded absolute file path, I still was unable to send the default MP3 to the client.
I decided (discovered) that one way to send a default MP3 when the requested file is missing is to use mod_rewrite for all requests of MP3s. I added the below lines to my htaccess file :
RewriteEngine on
RewriteRule (.*).mp3$ mp3download.php?file=/$1.mp3
mp3download.php checks for the existence of the file, and if the file is not found, the default is served.
Thanks again for the info.