There is a @todo in \Drupal\Tests\media_library\FunctionalJavascript\MediaLibraryTestBase::pressInsertSelected to consider adding screen-reader assertions every time the method is called. That should be implemented in a patch and reviewed to confirm it is an effective addition.
The other methods in \Drupal\Tests\media_library\FunctionalJavascript\MediaLibraryTestBase should also be evaluated to see if any of them result in consistent screen-reader behavior. If so, screen-reader assertions should be incorporated into those methods as well