you have to create a generic Elastica\Query() object. Then you can add sort to this query with ->addSort($sort)
And later you can assign a proper query with ->setQuery();
Your example should look like this
$query = new \Elastica\Query();
$query->addSort(array('price' => array('order' => 'asc')));
$q = new \Elastica\Query\QueryString($searchterm);
$term = new \Elastica\Filter\Term(array('isactive' => true));
$filteredQuery = new \Elastica\Query\Filtered($q, $term);
$query->setQuery($filteredQuery);
$players = $this->get('fos_elastica.finder.xxx.player')->find($query);
Boost allows you to make one field more\less important than other within a query.