Unhappy people with an inner voice have a grumpy crank personality as their inner voice. Taming and neutralizing that grumpy crank is the entire point of the Cognitive Behavioral Therapy school of Psychology. They have a process called an Inner Dialogue Audit where a list of 10-20 simple questions are given, and you ask yourself these questions. If you answer "yes" to any of them, then your inner voice has a bias, is effectively lying to you about your moment to moment observations and thoughts. And once this bias is identified, it evaporates because deception known no longer deceives.
I guess the inner voice is what one strives to just observe and let be while meditating. After a good meditation session I feel less mentally tense (eg not clinging on to superficially important todo items).
Seems like inner voice is related to mental state in a major way. I have pretty severe adhd and am unmedicated so my inner voice is more like an old radio inbetween stations (= chaotic).