> Do most people mean "multiple voices" when they say "inner monologue" ?
You will find different conceptualizations of this phenomena, but generally speaking it is very prolific to treat mind (term used imprecisely) as comprised of many sub-parts (sub-personalities/voices/selves/states). You can treat them as separate, functional patterns of behavior with their own agendas, feelings, beliefs, etc. The fun part is that you can engage them and talk to them, eg. you can talk to your Inner Child (you will feel it as separate and feel its child like nature) or your Lazy part or any part you recognize as a bigger pattern. You can even talk to your Sorrow, Silence, Energy of Being (as opposed to doing), etc. You can talk to parts of yourself which were dormant or disowned/repressed (eg. Anger, Romantic Self,...). You can talk to your demonized, aggressive Inner Critic.
The changes which occur due to shift in perspective which results from such dialogues are... beyond the scope of this comment. ;)
edit: Seems I am late for the party, but definitely for a treat here (including the article). (I do Voice Dialogue for a living, which is a practical method of meeting those different parts of ourselves.)