Hi,
the manual sort is a bit different to the sqlite sort where you can sort on multiple fields. If you want to sort the files manually you can use an existing field such as the Tagline which is not used for music and put a value in there then select to sort the playlist by Tagline and that will give you the order you want. So the Tagline would have 1 for the first song and 2 for the second and so on.