How to parse this shortcode?

shortcode_parse_atts will give you managable information.

  $atts = shortcode_parse_atts(
    '[soundcloud url="http://api.soundcloud.com/tracks/67658191" params="" width=" 100%" height="166" iframe="true" /]'
  );

You will get an array that looks like this:

array(7) {
  [0]=>
  string(11) "[soundcloud"
  ["url"]=>
  string(41) "http://api.soundcloud.com/tracks/67658191"
  ["params"]=>
  string(0) ""
  ["width"]=>
  string(5) " 100%"
  ["height"]=>
  string(3) "166"
  ["iframe"]=>
  string(4) "true"
  [1]=>
  string(2) "/]"
}

You can the do something like:

  var_dump($atts);  // dump everything
  echo $atts['url']; // echo just the URL

I don’t understand the context you want to use this in. I think you’ve left out a fair bit of explanation.