The N95 will not play FLAC files, you will need to convert them to MP3 or AAC.
FLAC is Free Lossless Audio Codec. It's a very popular audio format right now, i rip all my CD's to to FLAC and use them with my Squeezebox 3. FLAC files are exactly the same quality as the CD they were ripped from, as no data is thrown away to make the files extra small. However, FLAC files are very large (an album will typically be 300MB+) and not suitable for portable devices like the N95. The sound quality and features of the N95 would not justify the use of a lossless codec anyway. Keep the FLAC's, but convert to 192-320MP3 for your portable devices (and use the LAME MP3 codec). A great program called DBPowerAMP will do the trick.