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What is the best app to convert FLAC files to MP3?
Not sure which app, but Majorgeeks is loaded with free software, I'm sure there is a good program there to convert FLAC to mp3.
I myself would make mp3s (from FLAC) in the 256k or 320k range.
Probably this one: http://lame.sourceforge.net/
I used Sound Converter (or Sound Konverter, the German version) which is open-source (free) for use on Debian Linux (Ubuntu 16.04 LTS)