There are a lot of sketchy “audio converters” out there trying to get you to pay $20 for something you can do for free with this hidden feature in iTunes. It’s simple to use and works with your pre-existing library.
Open up the iTunes preferences (iTunes > Preferences, or Command+Comma) and navigate to the “General” tab. Click the “Import Settings” button at the bottom.
This window lets change the format in which new songs are added to your library. You can choose from any of the options here, but we’ll use MP3.
By default, the bitrate is quite low, but you can turn it up:
The bitrate directly controls the quality of the audio. 320kbps is as high as most MP3s go and is very good quality. However, if the file you’re converting isn’t the same quality, it won’t make a difference.