Copy IPhone email music attachments to my Apple Music Library

Sorry, that is not possible. There is no way to add music to the Music App from an email.

You would need to add it to a music library on a Mac or Windows PC and then sync it to the iPhone from there.

For Mac:

click here ➜ Import music from a computer to Music on Mac - Apple Support

then here ➜ Sync music between your Mac and iPhone, iPad, or iPod touch - Apple Support

For Windows PCs

click here ➜ Import media from a computer to iTunes on PC - Apple Support

then here ➜ Sync iTunes content on PC with your devices - Apple Support