Audio getting stripped in testimonial body

Just so happens there’s another filter available for you from the plugin. By default, WP strips out all shortcodes when using get_the_content() function. By wrapping it in do_shortcode(), it’ll work.

function custom_testimonial_the_content( $shortcode_output, $options ) {

	$get_content = wpautop( get_the_content() ); 

	$shortcode_output = '<div class="bne-testimonial-description">';
		$shortcode_output .= do_shortcode($get_content);
	$shortcode_output .= '</div>';

	return $shortcode_output;
}
add_filter( 'bne_testimonials_the_content' , 'custom_testimonial_the_content', 10, 2 );