To block the .mp3 file type in Easy Digital Downloads, we modify the edd_protected_directory_htaccess_rules filter and remove mp3 from the FilesMatch line

gistfile1.php

<?php

function edd_custom_modify_htaccess_rules( $rules, $method ) {
	
	switch( $method ) :

		case 'redirect' :
			// Prevent directory browsing
			$rules = "Options -Indexes";
			break;

		case 'direct' :
		default :
			// Prevent directory browsing and direct access to all files, except images (they must be allowed for featured images / thumbnails)
			$rules = "Options -Indexes\n";
			$rules .= "deny from all\n";
			$rules .= "<FilesMatch '\.(jpg|png|gif|ogg)$'>\n";
			    $rules .= "Order Allow,Deny\n";
			    $rules .= "Allow from all\n";
			$rules .= "</FilesMatch>\n";
			break;

	endswitch;

	return $rules;
}
add_filter( 'edd_protected_directory_htaccess_rules', 'edd_custom_modify_htaccess_rules', 10, 2 );