NPCs are not soulless automatons

NPCs are egregore-controlled humans, they still have an independent mind but their mind is enslaved and used as a tool for expressing the egregore mind, like a two-way radio channel.
see https://en.wikipedia.org/wiki/Egregore
Its possible to disrupt the link to egregore by subversive thought insertion(aka memes), allowing the original mind to assert its will and independent thought. The egregore can't focus on multiple mental challenges simultaneously and the "radio channel" has limited capacity.
NPCs can be reformed to PCs.

The core idea might not be in the inner monologue absence, i suspect NPCs just can't attribute thoughts to their source:
When a NPC gets a thought, the NPC assumes that the thought is being created by his own will and not from a process in the mind or from external influences.
The enables programming to bypass "attribution check" that judges information sources - as the NPC believes all thoughts are coming from its own I-mind.
Inner monologue however helps to put the distinction from conscious thoughts(coming from willful contemplation on a topic) and generated thoughts(coming from memory, external influences/senses, background processes).
Without the inner monologue every thought is read with the same non-verbal mental recall circuit that lacks "color" of thought: voices add "color"/"temperament" which helps to distinguish between sources of thoughts.

Here is catch: Inner monologue doesn't guarantee that the person can distinguish the source of voices/thoughts, he can be assuming each voice is the same channel with different topic. Its a skill that has to be learned or developed. Discriminating mind can "entertain a thought without accepting it." because it knows that the thought isn't coming from its will.