OPDS audiobook manifest order is wrong

Often the Librevox audiobook manifests are missing files from their readingOrder
https://api.archivelab.org/books/kniga_zitij_svjatyh_na_mesjac_avgust_eu_0811_librivox/opds_audio_manifest is totally empty

https://api.archivelab.org/books/short_poetry_087_1004_librivox/opds_audio_manifest ought to have 23 poems as per https://archive.org/details/short_poetry_087_1004_librivox, but it only has one -- the sixth one

Start by reproducing the two reported /books/.../opds_audio_manifest responses and compare them with the corresponding Archive.org audiobook entries and readingOrder. Done means the first manifest is populated and the short-poetry manifest includes all 23 poems in the correct order.

Written by the indexing model from the issue text.