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-
