If the files are not present in iTunes library, drag-and-drop them into iTunes and then select them. Now, go to the Music list (click the music icon in the top left corner of iTunes).Click OK and OK again, to close Preferences.(Select Custom if you want to specify more options).
In the next drop-down choose the bitrate for your mp3s.Now, click on the Import Settings button.Go to iTunes Preferences and click on the General.If you are trying to convert an audio recording made with QuickTime or any other m4a files to MP3s or AACs on a Mac, you can do it using iTunes.