I don’t know.
Okay, I had to go and look it up...
Bit Rate: Where possible, we encode our MP3 files using variable bit rates for maximum audio quality and smaller file sizes, aiming at an average of 256 kilobits per second (kbps). Using a variable bit rate allows us to allocate a higher bit rate to the more complex sections of music files while using a smaller bit rate for the less complex sections. The average of these rates is then calculated to produce an average bit rate for the entire file that represents the overall sound quality. Some of our content is encoded using a constant bit rate of 256 kbps. This content will have the same excellent audio quality at a slightly larger file size.
http://www.amazon.com/gp/help/customer/display.html?nodeId=200154210
Here’s something in a comparison of ... “MP3 vs AAC vs FLAC vs CD”
—
Quote: “So the best formats to use are the lossless ones, in case you were wondering, but if space is an issue, then use 320 Kbps AAC. MP3 comes from the MPEG-1 codec and AAC comes from the MPEG-4 codec, so the takeaway lesson is avoid MP3 if possible, even 320 Kbps MP3. It simply doesn’t compare.”
[ http://manufacturedenvironments.com/2008/06/mp3_vs_aac_vs_flac_vs_cd/ ]
—
MP3 plays on iPod, AAC plays on iPod, but FLAC doesn’t play on iPod. However, you can use Apple “lossless” if you want, but that would be overkill, I would think, on a portable device... :-)