The mp3 playback is awful. All the reviews i've seen (and there are loads) completely miss this point. I'm sure most look at the specs and don't actually listen to the thing. It sounds as though there is aliasing around the mid frequencies. Particularly evident on vocals. I thought my unit must be faulty, but turns out that they are all like that. Does anyone know of any firmware update (not sure if thats possible) or advice?

You really get what you pay for, I suspect. These things are OK for travelling on the train or in busy areas where you just want to block out surrounding noise with something more enjoyable, but if you've got a good ear for music quality, then for decent MP3 decoding hardware you have to pay quite a lot more. I doubt a firmware upgrade is available and I doubt even more that it would make much difference.

SOME ARE FAULTY!!!!!!!
I have gone through two of these Napa 311 devices before they sent me one without this problem.
In total it has cost me an extra £20 to send them back what with all the phone calls and what have you. I am sure that the second one that was sent to me was previously returned by someone else so I went to the trading standards. A word from them and I got sent a NEW one that WASN'T faulty.
DO NOT SETTLE FOR FAULTY GOODS, MATE.
I was, at first, fobbed off by being told that I had to use CCITT de-emphasis (or something) I eventually found a way to set that using a piece of shareware, but it didn't help at all. Then they told me my mp3 player only supported mp3s with a 16KHz cut-off, that is also NOT TRUE. Don't be fooled by retailers, give them as much hassle as it takes to get YOUR way.
Whatever happened to "the customer is always right"?