How many variables can you eliminate?
If you want to compare FLAC and MP3 audio quality effectively you are going to need a properly designed and controlled experiment. If you can get some known good source recordings and a studio monitor setup you could then encode the files in various formats and conduct blind tests with several listeners. Of course audio quality is a highly subjective thing as evidenced by claims to "hear" a difference between two cables carrying a digital signal, but blind testing should filter out bias.
Biting the hand that feeds IT
