I downloaded flac files from my PC to the App folder of my iPhone via iTunes. When I open VOX, it shows some songs but not others on the library. For example, when there are 12 songs on an album, only 10 songs appear on the library. I re-checked the VOX folder on the iPhone and confirmed the flac files of the missing songs are there.
I do not see a pattern on the missing songs, but it seems that it happens more often on Japanese titles songs. Furthermore, it seems to occur randomly–I was able to see all 12 songs right after I downloaded the songs, but next day I see only 10.