It appears that flac to mp3 conversion is related to trasnscoding of FLAC compressed audio files to common MP3 audio, something that can be nowadays done using almost any decent multimedia or audio conversion program. A lot of users are also looking for a way to convert flac to mp3 online which several websites offer as part of online audio converter, however the end result may wary, but overall they should work as they advertise.
Just keep in mind that FLAC (free lossless audio codec) audio format is lossless and MP3 (MPEG Layer 3) is lossy, converting will decrease the final audio quality, but the resulting file will be much smaller in file size.
Open FLAC file Open MP3 file
Audio converter for Windows and Mac
A multimedia converter
Audio conversion utility
A free audio converter
A set of audio and ripping utilities for Windows
A professional digital audio editor
One of the many FLAC audio converters
Universal video converting software for Mac
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
A dedicated audio converter
An open source audio conversion utility
An audio synth software