The excerpt will only fetch the text, triming tags. You need to make a new custom field (you can use Advanced Custom Fields) for the ID of that iframe. Then, you need to edit your content_excerpt.php (I don’t know wich theme are using) and fetch that field instead or below your excerpt.
For example: (using Advanced Custom field and Twenty Thirteen theme)
<?php
// fetch my custom field call it "player"
$player = get_field('player');
?>
<article id="post-<?php the_ID(); ?>">
<div class="entry-summary">
<h3 class="entry-title">
<a href="https://wordpress.stackexchange.com/questions/116747/<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
</h3>
<?php the_excerpt(); ?>
<iframe src="https://w.soundcloud.com/player/?url=http://api.soundcloud.com/tracks/<?php echo $player;?>&show_artwork=true&maxwidth=620&maxheight=930&wmode=transparent"></iframe>
</div>
</article>
Hope you understand my english 😀