Atrac is an audio format used by older Sony stereo systems and portable music players. This format has been largely forgotten, but if you have a Sony Atrac device, you'll need to convert MP3 files to the Atrac format before you can upload them to your music device.
Download and install an audio converter program such as one found at AllConverterTools.com or MP3-CD-Converter.com. These three sites all feature audio converter programs that function similarly, and they convert MP3 files to the Atrac format.
Launch the audio converter program onto your computer and click the "Add File" button on the program screen. Select the MP3 audio files you want to convert.
Repeat the file selection process until you have selected all the audio files you want to convert.
Select "Atrac3" from the format option pull-down menu at the side of the screen, then choose where you'd like the converted files saved to.
Click "Convert" to change the MP3 audio files to the Sony Atrac format.