Custom code

<?php
/**
 * Custom MP3 Player theme function.
 *
 * @param theme
 *    Always set to 'mp3player'.
 * @param player
 *    The name of the player you want to use (default is 'Default').
 * @param url
 *    The URL to the mp3 file.
 * @param album (optional)
 *    The name of the album.
 * @param artist (optional)
 *    The name of the artist.
 * @param description (optional)
 *    A description of the mp3.
 * @return A WordPress Audio Player instance.
 */
theme($theme, $player, $url, $album, $artist, $description);
?>

Two examples:

<?php
print theme('mp3player', 'Default', 'http://www.example.com/audiofile.mp3', 'An example file', 'John Smith', 'An optional description of the file');
?>

Or if your MP3 file has embedded ID3 data:

<?php
print theme('mp3player', 'Default', 'http://www.example.com/audiofile.mp3', NULL, NULL, 'An optional description of the file');
?>