Let’s talk about permissions here.
The user that mt-daapd aka firefly is running as has different (lower) permissions than your user account.
For instance it cannot peek into your ~
Normally this user account is nobody/nobody, so you need to do either:
a.) put your music in a directory that nobody/nobody can access (that is read+execute permissions on the directory, read on the files)
b.) change permissions of said directory so that nobody/nobody can use it (+rx on directories, +r on the files)