It seems that the only way to convert Sony ATRAC audio files (.atrac) to MP3 (or any other multimedia format) is to use Sony SonicStage program which can be downloaded on Sony website. As far as we know, this is only way to achieve the atrac to mp3 conversion.
Convert Sony ATRAC audio to Compressed audio and music file, mp3 songs or ringtones.
We found one software available in the database for this conversion.
Microsoft Windows software - convert atrac to mp3 on Windows
Audio data management program for portable devices
Convert from atrac file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.