it's just a non-standard header that FLAC is skipping over.
In WaveLab, there is a setting that you can switch to use "optimized" headers. if you check that, you won't get a standard header, and you'll see that message when compressing to FLAC. However, if you uncheck that choice, WaveLab will create a standard header and you won't see that message again.
Also note that every file saved from the M-Audio MicroTrack has extra junk in the header. So if you compress a file straight from the MicroTrack, you'll also see that message.