I recently worked on a documentation project that required posts to be sorted alphabetically by title. The process can be done by using pre_get_posts() and the is_archive() conditional tag.

/*
 * Order post archives by title in descending order
 */
add_action( 'pre_get_posts', 'jd_archive_sort_alpha_title'); 

function jd_archive_sort_alpha_title($query){

	if(is_archive()) {

		// Set the order to ascending order
		$query->set( 'order', 'ASC' );

		// set the orderby to title
		$query->set( 'orderby', 'title' );

	}

};

As always if you want to use this plugin on your website test it first in a safe environment to verify that it doesn’t create a conflict. Happy hacking. 🙂