How to use Subtitles/Captions with MediaElement.js module
The MediaElement module doesn't support it. In order for ME.js to display subtitles, you need to add a &lt;track&gt; element (e.g. &lt;track kind="subtitles" src="subtitles.srt" srclang="en" /&gt;). The module's theme function, theme_mediaelement_video(), doesn't render that element. 

In lieu of this, you can override the theme function to add the required &lt;track&gt; element. For example placing this in your theme's template.php:

//Here we're assuming an .srt with the same name of the video exists alongside 
//(e.g. "sites/default/files/video.mp4 &amp; sites/default/files/video.mp4.srt)
//Given you might want to upload srt files along with content
// you'll want a better solution

function MYTHEME_mediaelement_video($variables) {
  $output = '&lt;div class="mediaelement-video"&gt;';
  $output .= '&lt;video ' . drupal_attributes($variables['attributes']) . ' &gt;';
  $output .= '&lt;track kind="subtitles" src="' . $variables['attributes']['src'] . '.srt" /&gt;';
  $output .= '&lt;/video&gt;';
  if ($variables['settings']['download_link']) {
    $output .= '&lt;div class="mediaelement-download-link"&gt;&lt;a href="' . $variables['attributes']['src'] . '"&gt;' . filter_xss_admin($variables['settings']['download_text']) . '&lt;/a&gt;&lt;/div&gt;';
  }
  $output .= '&lt;/div&gt;';
  return $output;
}