Transfer music files from old iMac to new iMac?

You did well by not migrating everything, but... the easiest solution would have been to use Migration Assistant but only elect to migrate the account(s) and not the applications.

Did you try just copying the whole Music folder?

What you need to do is copy the iTunes library folder

to wherever you want to store it. I have mine on an external SSD. When you open the Music folder for the first time it should ask you what library you want to import from and if you want to consolidate the library, i.e. copy the audio files into the new Music library location. I opted not to.

Audiobooks are now in the Books library and those must be kept in the default location, in the Home/Library folder.

Personally - open the Music Application >> File and pop down menu Import and choose From where they are located. Music should vacuum the music into the right location.

The Music folder is supposed to be in the exact same location - that is, inside your home folder - at the same level as Pictures, Movies, Documents...

Gosh it would seem less time consuming trying to get to the correct location to put the music than to just Import music and all in the right place bingo.