you wouldn’t need a flac file, you can use any wav file, the audio of both is identical.

regadring your question, you can think mp3 as the jpeg of music. both mp3 and jpeg use fourier transforms*. so, to image what mp3 is doing to the audio, you can see what jpeg does to images (spoiler alert, unless you are aggressively compressing it, it is not noticable)

(*jpeg actually use discrete cosine transform instead of fft, but it is similiar enough)