There are a lot of sketchy "audio converters" trying to pay $ 20 for something you can do for free with this hidden feature in iTunes. It's easy to use and works with your existing library.
Open the iTunes Preferences (iTunes> Preferences or Command + Comma) and navigate to the General tab. Click on the "Import Settings" button below.
In this window, you can change the format in which new titles are added to your library. You can choose from all options here, but we use MP3.
By default, the bit rate is very low, but you can turn it up:
 The bitrate directly controls the audio quality. 320kbps is as high as most MP3s and are of very good quality. However, if the file to be converted does not have the same quality, this is no difference.
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Now that we've changed the import settings, we can use the built-in Create Copy feature to duplicate a song. Since we chose MP3 as the file format, the encoder uses it when copying the song. This option can be found under File> Create MP3 Version.
This duplicates the file so that there are two files with the same name in your library. You can right-click one of them and choose "Show in Finder" from the drop-down menu to access the actual MP3 file.
You can convert as many songs as you like iTunes stores the copies in the same album folder so you can search the Finder for "Date Modified" or "Date Added" to select the new titles. Search for files that were all created at one time.
From here you can move it to another location or delete the old files.
If you need something better than iTunes or not If you only want to add your files to iTunes to convert them, you can try XLD (a free open source audio converter).
Just download the DMG, open the program, select the output format and choose "Open" from the File menu. The files are automatically converted and stored in the same directory.
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