FLAC is better than mp3 because it sounds better
I use headphones. FLAC sounds better than everything else because it is what it is and not some mathematical equivalent that strips anything out. I can't speak to the Sonos issue, but I can tell you that the musicians I know all prefer FLAC to Apple's supposedly "lossless" format and to all mp3, yes, even at 320k. It just sounds better in ways that are tough to articulate without sounding like a audiophile nutcase. It's more natural sounding. I don't care why and neither do they.