What’s the term for “hearing” words when you read or think?

It's called subvocalization:

Subvocalization (also known as auditory reassurance) is a very common habit among readers. It involves saying words in your head while reading and it’s one of the main reasons why people read slowly and have trouble improving their reading speed.
_{Speed Reading Tips: 5 Ways to Minimize Subvocalization}

Subvocalization is an excellent answer if you don't Actually Hear the words. If you DO hear the words, or perhaps feel them or smell them, the 'overlapping' of normal sensory input into non-ordinary perception (i.e. colors that have numeric value, etc) it's known as Synesthesia, and is far more common than those of us with it care to admit.