org-publish-sitemap-sort-files

org-publish-sitemap-sort-files is a variable defined in `ox-publish.el'.
Its value is

alphabetically

Documentation:
Method to sort files in site-maps.
Possible values are `alphabetically', `chronologically',
`anti-chronologically' and nil.

If `alphabetically', files will be sorted alphabetically. If
`chronologically', files will be sorted with older modification
time first. If `anti-chronologically', files will be sorted with
newer modification time first. nil won't sort files.

You can overwrite this default per project in your
`org-publish-project-alist', using `:sitemap-sort-files'.

You can customize this variable.