Although you can convert MP3 audio files to FLAC format, which is what mp3 to flac conversion represents, it won't be of the quality expected from a true FLAC. Lossless FLAC audio files are made directly from audio software authoring tools or original CD tracks and PCM audio files (.wav).
Open MP3 file Open FLAC fileAudio converter for Windows and Mac
A multimedia converter
A free audio converter
A set of audio and ripping utilities for Windows
A music library management program - renamed to Helium (version 12) and replaced in 2016
An audio player and converter
Universal video converting software for Mac
A multimedia player with conversion capabilities
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
A dedicated audio converter
An open source audio conversion utility
Video converting software
An audio synth software