Hi, Josi.  Thanks for joining the discussion.

   Indeed, I wouldn´t say that "my friend" is wrong, he was talking about a particular friend (the one that discussed the "voices in hall" vs "voices in the hall").   It might be

that he was talking to only one friend at that time.

   If there was more than a friend around, and one came up with the question, then he could said "a friend of mine" (um amigo meu - dentre outros, ou seja um dos meu amigos).

    A teacher of mine - um dos (meus) professores/ um dos professores meus.

    My teacher - meu professor (here I single out a teacher, say, if there was only one

teacher at the time I am talking about).   If there was more than one, and you want to specify wich, you can say My teacher of Maths - My Math´s teacher.

    Thus, when you say "a friend of mine"/"a teacher of mine" etc, we suppose that you have more than one friend/teacher etc.  And you are talking about one of them.

     Thus, it depends of context (what you intend to convey).

     I hope it has been clear, you know, I am still in the basics.