cdarev

Maybe this can help you. It's the official Sony Atrac to MP3 conversion tool. The bad is there are specific versions for the many Sony devices, but it's worth a try:

http://www.sony.co.uk/support/en/topics/mp3-conversion-tools

Also there are some old topics here:

http://avic411.com/index.php?/topic/474-at3-files-what-are-they-what-have-you-tried/

http://avic411.com/index.php?/topic/8263-at3-converter/

http://avic411.com/index.php?/topic/9252-anybody-changing-at3-to-mp3-or-vice-versa/

and this one is part of other topic:

http://avic411.com/index.php?/topic/456-ok-i-got-my-hd-to-read-now-what/page-8

Any of you who has backed up .at3 files could try.

p.s. I don't possess Z series device. My Interest about the topic: I want to port/move CD ripping software from Z series to the F series.