How do we hear ourselves think?
When we speak out loud there are two signals that the brain receives: the internal monologue and (assuming you are not deaf) auditory feedback from our speech output. This may be why we tend to think we are speaking louder than we actually are. Your brain also receives a 3rd feedback signal from the articulation/speech muscles. Your brain learns to associate all 3 signals with each other (thinking a word, with the sound of the word, and the movement of the speech muscles needed to speak the word).
So if you are thinking internally, your conscious brain will only receive the internal monologue signal.
But as I\we mentioned, the internal monologue is extremely closely related to the production of speech (this topic is known as “embodiment”). Studies show that when people read text silently, muscles used for speech are slightly activated. Also, speech production areas of the brain are activated when people speak internally. This all suggests thinking and speaking use similar neural pathways, and so the brain perceives thinking almost like speaking.
Note: this is referring to clear-cut intended internal speech. More ambiguous “thought” is going to be much more complex.
TL;DR: The act of speaking, the sound of speaking, and the internal thought which is correlated with the speech, are all highly associated with each other (They use similar brain pathways). So when you “hear” yourself think, it is probably because you are activating the same brain pathways used for normal speech and hearing.