Open the audio conversion utility application. Click "Add Files."
Navigate to the .au file. To assist filtering the music files, select the ".au" file type from the File Type drop-down menu on the "Add Files" dialog box. Select the desired file.
Choose the folder that will receive the output. Click on the "Browse" button and navigate to the desired folder.
Select the output file type. Select ".mp3" from the drop-down menu in the lower left corner of the application window.
Click the "Convert" button to begin the file conversion process.
Click the "Open Output Folder" button or open Windows Explorer and navigate to the output folder. Verify that the new .mp3 file has been converted and saved.