Files play directly from your device storage — nothing is uploaded or re-encoded, so quality stays exactly as the original file. Turn on "keep audio on", then lock your screen: playback continues and you'll get play/pause/next controls on your lock screen. Note: your OS may still pause background tabs for battery saving on some phones/browsers — if that happens, add this page to your home screen once, then reopen it from there for the most reliable background audio.