I use TuneClone for conversion. (Most) Songs purchased on iTunes are copy protected (AAC) and can’t be converted without 3rd party software. This product does great batch conversion and is quick and easy. Worth the investment if you’re doing a lot.

http://mac.tuneclone.com/

This is not true anymore. Of the hundreds of songs my daughter has purchased, 3 are protected, and those were the first 3 she purchased prior to itunes selling unprotected songs. And the protected files have the file extension m4p while the non-protected files have the extension m4a, which is what the op is interested in. That being said, it could still be a good way to convert.

Ron