MIDI files contain notes and instrument instruction for a playable song. As such only special audio editing / conversion software with proper instrument libraries and translation capabilities can convert MIDI files to actual audio files like the FLAC (free lossless audio codec). So it is not much of conversion, more like generating *.flac audio file from *.midi source.
Convert MIDI-sequention sound to Audio files encoded by Flac - free lossless audio codec.
We have found 2 software records in our database eligible for .midi to .flac file format conversion.
Microsoft Windows software - convert midi to flac on Windows
An audio editing software for Windows and Mac
Convert from midi file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned midi to flac. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A software synthesizer for Windows