It is important here to place you iframes in <div class="flex-video">...</div> This is a feature from Foundation 5 framework and it makes iframes responsive and well positioned.

SoundCloud

Code used:

<div class="flex-video">  
    <iframe src="https://w.soundcloud.com/player/?url=https%3A//api.soundcloud.com/playlists/2818492&amp;auto_play=false&amp;hide_related=false&amp;show_comments=true&amp;show_user=true&amp;show_reposts=false&amp;visual=true"></iframe>
</div>  

Vimeo example

Bob Sinclar "Cinderella" (She Said Her Name) from Alexander Tikhomirov on Vimeo

Used code:

<a href="http://vimeo.com/75911644">Bob Sinclar "Cinderella" (She Said Her Name)</a> from <a href="http://vimeo.com/stikhomirov">Alexander Tikhomirov</a> on <a href="https://vimeo.com">Vimeo</a>

<div class="flex-video">  
    <iframe src="//player.vimeo.com/video/75911644"></iframe>
</div>