Bipolar Disorder with Psychotic Features: This is the most likely diagnosis because mania with hallucinations is a classic presentation of bipolar disorder, particularly during manic episodes. The presence of hallucinations indicates psychotic features, which are common in severe manic episodes.

Schizoaffective Disorder: This disorder combines symptoms of schizophrenia (such as hallucinations) with mood disorder symptoms (like mania). It's a likely diagnosis if the hallucinations occur independently of the manic episodes.
Substance-Induced Psychotic Disorder: Certain substances can induce manic-like symptoms along with hallucinations. This diagnosis should be considered if there's a history of substance use that correlates with the onset of symptoms.
Major Depressive Disorder with Psychotic Features: Although less common, severe depressive episodes can also present with psychotic symptoms, including hallucinations. The presence of mania might suggest a bipolar component, but the primary mood episode could be depressive.

Neurosyphilis: This condition can cause a wide range of psychiatric symptoms, including mania and hallucinations. It's crucial to rule out neurosyphilis due to its treatable nature and severe consequences if left untreated.
Temporal Lobe Seizures: Seizure activity in the temporal lobe can manifest as psychiatric symptoms, including hallucinations and altered mood states that might resemble mania.
Wilson's Disease: This genetic disorder leads to copper accumulation in the brain and can cause psychiatric symptoms, including mania and hallucinations, along with neurological signs.

Cerebral Vasculitis: Inflammation of blood vessels in the brain can lead to a variety of neurological and psychiatric symptoms, including mania and hallucinations, though this is exceedingly rare.
Prion Diseases (e.g., Creutzfeldt-Jakob Disease): These rare, degenerative brain disorders can present with rapid onset of psychiatric symptoms, including mania and hallucinations, along with neurological deterioration.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by encephalopathy and a range of psychiatric symptoms, including mania and hallucinations.