Open iTunes.
Pick the track that you would like to convert into the AIFF format file. Converting more than one track at a time takes much less time if you create a new playlist. Add into the playlist all of the files that you would like to convert into AIFF files. This is especially handy if you are going to convert files from different folders or albums.
Right-click (if using a PC) or control-click (if using a Mac) on the audio files that you would like to convert and choose the "Create AIFF Version" option. Upon clicking this, your files will start to convert.
Find the freshly converted AIFF in the music library. If the settings are not changed, each of the converted files will be beneath the original.