Your brain is mis-firing, by attributing inner voices (that no one else can hear) to outer sources. This link covers the medical reasoning behind that: Impaired Self-Monitoring of Inner Speech in Schizophrenia Patients with Verbal Hallucinations and in Non-clinical Individuals Prone to Hallucinations
I know what it’s like, just had a series of auditory hallucinations during my trip to the corner store. If I had confronted the guy saying the nasty things, he wouldn’t have had any idea who I was.