320kbps MP3 is enough:
For most listening purposes, with most equipment and most ears, 320kbps MP3 is perfectly good enough. Sure, use FLAC for archiving purposes - it makes sense to have a lossless master of your CD on a server, NAS or PC in the house - but if you're clogging up a portable player with FLACs or Apple Lossless files, then either a) you have superior ears, a headphone amplifier and seriously high-end headphones or b) you're lacking in common sense.
192kbps might be enough:
Many of us simply couldn't tell the difference between 192kbps and FLAC. There are caveats. A wider selection of music might have thrown up more differences, and some ears and some headphones might reveal more obvious distinctions. That said, there's no reason why you should turn your nose up at 192kbps or 256kbps files if you're just looking for something to listen to in the car or on the daily commute. You'll get more tracks on your player, and the sound quality will probably be good enough.
Audiophiles might not be able to spot the difference, but they can come up with a more convincing rationale:
There was no convincing evidence that those subjects who consider themselves audiophiles and have invested more in high-quality MP3 players or headphones could discern lossy from lossless audio files better than anyone else. What they could do, however, was give you more convincing arguments as to why one version sounded better than the other. In effect, they had better tools with which to convince themselves that their subjective impressions were correct, even when those impressions were entirely misleading. Don't forget this when you make your own, non-blind comparisons. Maybe that FLAC file really does sound better than the 192kbps MP3, but are you quite sure that you're not just listening for factors to convince yourself it does?
Look after your hearing:
The only person to get all four tracks right is someone who listens to their headphones at pitifully low volumes and hasn't attended any rock concerts. We can think of two explanations. One, the subject has particularly sensitive ears, so doesn't need to turn the volume up high. Two, the subject hasn't wrecked their hearing through years of listening to a walkman/MP3 player at high volumes and/or seeing Motorhead at the Hammersmith Odeon. Arguably, both apply.