two custom fields question

All you have to do to hide this link is to check, if it’s empty:

<?php if ( $link = get_post_meta($post->ID, 'mp3_link1', true) && trim($link) ) : ?>
    <a href="https://wordpress.stackexchange.com/questions/326188/<?php echo esc_attr($link); ?>" target="_blank">MP3</a>
<?php endif; ?>