Thanks to @kero I have implemented this code:
<?php echo do_shortcode('[responsivevoice voice="Italian Female" buttontext="Ascolta"]'.'<br>'.$bodyContent.'[/responsivevoice]'); ?>
The $bodyContent variable contains the HTML value of the article’s body, obtained using this function: http://www.web-templates.nu/2008/08/31/get_the_content-with-formatting/index.html
<?php $bodyContent = get_the_content_with_formatting(); ?>