What are some reasons why visual and auditory hallucinations would start suddenly in an elderly person?

Delirium. Delirium is a medical emergency associated w/high mortality if underlying cause is not treated. It is defined by sudden onset of hallucinations (audio, visual or both) & delusions w/fluctuating states of consciousness & awareness. In other words, patient all of a sudden starts acting crazy. This can be due to pain, low oxygen, drugs, salt/mineral imbalance, kidney/ liver failure, heart attack, etc.