People who use Apple Music run into the inability to edit m4p (protected) files. Here is your guide to understanding what is going on. Apple has has two music file formats:
m4p (protected), and
m4a (not protected).
If you bought a song on iTunes before 2009, it was in m4p (protected) format. If you bought a song after 2009 on iTunes, or used their sync system to convert m4p into m4a files, then it is in m4a (not protected) format.
However, if you create an offline copy of a song via Apple Music (streaming service), then THAT is m4p (protected) format. So if you have made an “offline” copy of a song via Apple Music, you need to delete that copy (the m4p protected file) before you can download (or re-download) the purchased m4a (not protected) file.
If you don’t want to have to deal with the hassle of figuring out if you have a protected or unprotected version of the song, and don’t want to have to know the difference between m4a and m4p files, I recommend NOT using Apple Music for streaming, and use Spotify instead. That keeps the difference between what you’ve bought and what you are streaming nice and SEPARATE.
iTunes is non-intuitive when it comes to converting a song from one format to another.
iTunes can convert audio into any of five different formats, AAC (which has an m4a extension), AIF, Apple Lossless, MP3, and WAV. But right-clicking on a song only shows you one choice to convert to, and your conversion option is only whatever you have your CD import settings set to! (It doesn’t really make a lot of sense, but it is what it is.) To change it to MP3, just go to Preferences -> General -> Import Settings, and select MP3 Encoder.
When you do, be SURE to set your MP3 import settings to:
• Stereo Bit rate: 256 kbps
• [check] Use variable bit rate encoding (yes)
• Quality: Highest
• Sample Rate: 44.100 kHz
• Channels: Stereo
• Stereo Mode: Normal
• [check] Smart Encoding Adjustments (yes)
• [check] Filter Frequencies Below 10 Hz (yes)
If later, you need to convert music to a different format that iTunes supports, then follow the steps listed above, but select the destination format that you want to convert to. For example, if you need a WAV file, then select WAV in your Import Settings, and once you do that, Convert to WAV will be an option when you right-click on a song.