ripped CD (wav) ignored by Twonky

[habbah]

Hi

system set-up:
hardware: PC with Windows 7, NAS QNAP 409, T&A streamer - all connected via ethernet
software: Twonky Media and Twonky Media Server

Today I installed the trial version of dbpoweramp to rip CDs to wav files. The ripping went fine but Twonky (and also Windows Media Player while double checking) keeps ignoring the files, i.e they do not show up in their libraries.
I ripped CDs on the PC and moved them over to the NAS and alternatively I executed a ripping process directly on the NAS. I did restart all hardware afterwards and I rebuilt the data base of the NAS as well. ( I can also tweak the Windows Media Player to run the newly ripped wav files so they must have been ripped correctly.)
Is there someone who has an idea how to tackle this annoying issue?
In the past I used Ripstation Micro which worked together with Twonky but with which I had issues re the saved metadata.
Many thanks in advance //hb