To add a new “Video” field to Speaker you could use this plugin:
http://wordpress.org/extend/plugins/types/
But please remember we can’t provide support for external plugins.
Then you should edit the “template-speakers.php” file to show the video embed in place of the image.
Here you can find instructions about video embedding:
http://codex.wordpress.org/Embeds
http://codex.wordpress.org/Function_Reference/wp_oembed_get