Is the encoding of the MP3 affected battery time as well in not only PDA but iPod. There are many factors here from how old the PDA battery is, temperature of the components and even which charging system they used. Even the SD card used can affect the results wildly.
Sorry. One has to take the numbers as a guide and not what you'll get.
When i read on Cnets reviews, they said they got five hours of MP3 out of the Zire 31, i get around 8 or 9, do they leave the display on? or do they use realplayer and let the screen turn off.