Metabox

/* Custom Meta Box */

add_action( 'add_meta_boxes', 'cd_meta_box_add' );
function cd_meta_box_add()
{
    add_meta_box( 'my-meta-box-id', 'Music', 'cd_meta_box_cb', 'page', 'side', 'low' );
}

function cd_meta_box_cb( $post )
{
    $values = get_post_custom( $post->ID );
    $text = isset( $values['my_meta_box_text'] ) ? esc_attr( $values['my_meta_box_text'][0] ) : '';
    $selected = isset( $values['my_meta_box_select'] ) ? esc_attr( $values['my_meta_box_select'][0] ) : '';
    $check = isset( $values['my_meta_box_check'] ) ? esc_attr( $values['my_meta_box_check'][0] ) : '';
    wp_nonce_field( 'my_meta_box_nonce', 'meta_box_nonce' );
    ?>

    <p>
        <select name="my_meta_box_select" id="my_meta_box_select">
            <option value="none" <?php selected( $selected, 'none' ); ?>>None</option>
            <?php $audios =& get_children( 'post_type=attachment&post_mime_type=audio/mpeg' );
                foreach ( (array) $audios 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
}

add_action( 'save_post', 'cd_meta_box_save' );
function cd_meta_box_save( $post_id )
{
    // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    // if our nonce isn't there, or we can't verify it, bail
    if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'my_meta_box_nonce' ) ) return;

    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post' ) ) return;

    // now we can actually save the data
    $allowed = array(
        'a' => array( // on allow a tags
            'href' => array() // and those anchords can only have href attribute
        )
    );

    // Probably a good idea to make sure your data is set
    if( isset( $_POST['my_meta_box_text'] ) )
        update_post_meta( $post_id, 'my_meta_box_text', wp_kses( $_POST['my_meta_box_text'], $allowed ) );

    if( isset( $_POST['my_meta_box_select'] ) )
        update_post_meta( $post_id, 'my_meta_box_select', esc_attr( $_POST['my_meta_box_select'] ) );

    // This is purely my personal preference for saving checkboxes
    $chk = ( isset( $_POST['my_meta_box_check'] ) && $_POST['my_meta_box_check'] ) ? 'on' : 'off';
    update_post_meta( $post_id, 'my_meta_box_check', $chk );
}