A better code for no post format?

I’m not sure I understand. Do you just want to join all the if-statements together? If that’s the case you could use elseif, like so:

<?php
$format = get_post_format();
if ( false === $format ) {
echo 'This is a default post';
the_excerpt();
} elseif ( has_post_format( 'video' )) { ?>
   A VIDEO POST
    <?php the_content();
} elseif ( has_post_format( 'audio' )) { ?>
    AN AUDIO POST
    <?php the_content();
}
?>