FLAC files are very similar to .shn files. Both require a tool to convert to wav format.
SHN files can be converted to wav with the
mkwACT tool
(learn more by reading the
shn FAQ)
FLAC files can be converted to wav with
Flac Frontend (also available
here <- also contains pictures and a few tips on what to do should you get an error message)
more FLAC info
here
If you are interested in learning a bit more, Erich made a post a while back that you might be interested in reading
here