Hello,
Indexer doesn't extract alt text contents. You can use getDocxPathQueryInfo with a custom Xpath query to get that information:
$referenceNode = array(
'customQuery' => '//w:drawing//wp:docPr',
);
$contents = $docx->getDocxPathQueryInfo($referenceNode);
foreach ($contents['elements'] as $content) {
echo $content->getAttribute('descr');
}
Regards.