如何convert手机版的录音 .m4a to .mp3:
If the song is already in iTunes, it's easy to do it right within it. Go to the iTunes Preferences and choose "Import Settings" on the "General" page. You can set the desired output format here.
Afterwards, select the songs you would like to convert and choose Advanced > Create XY Version, where XY is the file format you specified in the Import Settings.
When iTunes has finished converting the files, you will have each track twice - you could use a smart playlist to filter the ones in the new format.