Missing volume_sorting?

There should be a metadata element named "volume". This is part of the default configuration. For this element (just like for any element) you can define xpath statements.

For us, these are set as followed:

XPath: ./mods:part/mods:detail/mods:number
XPath (Sorting): ./mods:part[@type="host"]/@order

OK, if that metadata element also is marked "Prepare for sorting?" and after subsequent reindexing all volumes seem to be in place finally, so thank you for pointing this out.

But where is that "default configuration"? It's quite impossible for anyone configuring DLF to figure this out if there's no real default, i.e. something being automatically set when installing the software.

Besides it isn't very logical that one should have a metadate called "volume" where one already most likely has a structure also called "volume" (which is DFG viewer compatible), and that this metadate is responsible for the particular issue we're talking about here.

The default configuration isn't set automatically, because then it would overwrite your individual configuration each time you update the extension. At least this is how the TYPO3 updating mechanism works when providing static default data.
But there is a backend module you could have noticed, which allows you to import the default configuration into a sysfolder. When preparing for an update it is recommended to always import the default configuration into any sysfolder and compare it to your individual settings. If you want to look at the defaults without importing them into the database you can find them in /modules/newclient/structures.inc.php and /modules/newclient/metadata.inc.php.

The existence of a structure named "volume" and a metadate of the same name is no duplication, because it handles completely different things:
1. The metadata configuration always refers to the MODS section (or whatever bibliographic format you use) while the structure configuration refers to the METS structMap. So if you have a structure type and a bibliographic metadate of the same name, you have to configure both. (Thus this isn't a DLF issue but a METS/MODS annoyance.)
2. The structure element named "volume" means, that you have structures of type "volume" in your METS files. So the DLF only reflects what you have written in your files and therefore can't be blamed if there is a metadate of the same name. Theoretically you can name your volumes anyway you want to avoid this (but then you won't be compatible with the DFG viewer profile).
3. The metadate named "volume" refers to the name of a volume and its counting. This is an element of MODS which is used for identifying volumes (which often don't have bibliographic titles) and bringing them in the right order.

Sorting in list view always works by metadata, so if you want to sort the items by any property, you have to configure it as a metadate.