FLStudio doesn't decompress the wave files when you first install the program?
After some research the IL website explains that: "Many .WAV files included with FLStudio are .OGG encoded .WAV files. These files require the Vorbis ACM CODEC ...
FL generators...what have you changed from FL12 to FL20 that could be causing this problem, i have also discussed this with Antelope Audio who took remote control of my pc and had a long look and they said it is an FL ...