m3u to flac conversion is actually not possible, because .m3u file is but a playlist and does not contain any audio data (like FLAC songs). At best you can locate the path to the actual audio files, but it's often relative to the folder meaning the audio files would be in the same folder.
Updated: June 29, 2023
Unfortunately, there is no detailed software record in the database that match this conversion.
M3U
Files with m3u extension can be usually found as playlists, typically for a group of .mp3 files or whole folders.
FLAC
Files with flac extension are used for audio files compressed with the FLAC, aka Free Lossless Audio Codec. FLAC files do not lose any quality with their compression as MP3 do, but they are generally larger. Most modern media players support playback of FLAC files.