i got some good news, i managed to solve the flac issue. here is how i did.
although it seem to be a general cowon user problem, i just managed to adapt the solution on my C3 lol.
by using command promt window, i have easily changed the .flac to .fla extension than the audio files worked automagically!
i also contacted with the colorfly support and he said he will send me an older firmware of it.
so i would like to thank all you guys for all your help. if you have any question about that issue don't hesitate to ask lol.