There is no WinAmp for the Pocket PC, and most likly never will. But there are many 'clones' of WinAmp such as withMP3. You might want to check it out, although I don't think it plays .flac files. I used it on my old ipaq 3630 before I sold it and it seemed alright. I remember turning the bass up seemed to make the music sound Terrible, but then again that was a long time ago so they might have fixed that problem now.
http://withmp3.citsoft.net/
Adam