Unfortunately, the django-sphinx module is not very thoroughly documented. One thing which I could not figure out is how to do sorting. I thought that I had to set some kind of keyword argument on the SphinxSearch instance... (I have to admit though that the sorting is indeed demonstrated in the project README.rst, I simply overlooked it...)
But the solution was much simpler – you can use order_by directly on the SphinxSearch instance.
Make sure that the selected sorting key is marked as a sortable attribute (e.g. sql_attr_uint or sql_attr_str2ordinal) in your sphinx.conf.