Metabox

// Metabox Content

function meta_content()
{
    $post = get_post_custom( $post->ID );
    $selected = isset( $post['meta_options'] ) ? esc_attr( $post['meta_options'][0] ) : '';
{ ?>
    <p>
        <select name="meta_options" id="meta_options">
            <option value="none" <?php selected( $selected, 'none' ); ?>>None</option>
            <?php $options =& get_children( 'post_type=attachment&post_mime_type=audio' );
                foreach ( (array) $options as $attachment_id => $attachment ) { ?>
            <option value="<?php echo $attachment_id; ?>" <?php selected( $selected, $attachment_id ); ?>><?php echo $attachment->post_title; ?></option>
        <?php } ?>
        </select>
    </p>
<?php }}

// Save Metabox Content

add_action( 'save_post', 'meta_save' );
function meta_save( $post_id )
{
    // Check for auto-save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

        // Check for content
        if( isset( $_POST['meta_options'] ) )
            update_post_meta( $post_id, 'meta_options', esc_attr( $_POST['meta_options'] ) );
    }