Are you trying to stream the FLAC files with Twonky or DLNA, or are you opening them in web2go with a player like VLC?
The web2go interface is an encrypted connecton so the encryption makes it slow for large files (which FLAC are). An easier option is to stream the FLAC files through the AccessBook interface which is available in FeaturesPacks.
http://www.highlevelbits.fr/index.php?option=com_content&view=article&id=605&Itemid=80&lang=fr
