Asterisk 1.8 appears to have changed the file structure for the sound files. The language code is now a top-level directory within /var/lib/asterisk/sounds/ and the previous path is now under that, instead of having the language code as an optional last component in the pathname. This includes English (en), meaning that Asterisk is first attempting to look at /var/lib/asterisk/sounds/en/ogm/ for the sound files recorded by PBXManager for the IVR.
This is mainly an issue when (like for me just now) I'm trying to set up an office that uses multiple languages for their voice prompts. PBX Manager obviously has no clue any of the languages other than English exist, and if I move the files where they're actually supposed to go, PBX Manager doesn't realize they exist at all, regardless of language.