There do exist dedicated transcription services and solutions that can "convert" the speech from an audio file, like the FLAC, and save the words in a text format (.txt mainly). It may not always be perfect and commonly requires manual editing by human user.
Convert Audio files encoded by Flac - free lossless audio codec to Simple text.
We have found one software suitable in our database for this conversion.
On-line services software - convert flac to txt on web
A Voice Search & Transcription Platform
Convert from flac file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.