Are you testing with a module and in particular module format that actually has a separate volume column? For formats that do not have a separate volume column (and for modules that do not make use of it), returning 0 here is the expected behaviour.

e.g. the MOD format (example: https://modarchive.org/index.php?request=view_by_moduleid&query=42560) does not have a separate volume column, while IT or XM (example: https://modarchive.org/index.php?request=view_by_moduleid&query=55761) do.

I'm sorry, that was my fault.
I was a bit tricked that OpenMPT shows default sample volume and expected to get those values.
I tested with non default volume values and those got correctly returned by the function call.

Thanks, and sorry again. Can be closed.