From the other comments:
1. I hated audio books before the monologue, and I still do after.
2. (from @echelon's post) I was quite imaginative before the monologue - but more in a sort of a fuzzy, visual way. I had entire fantasy realms just "visible" inside my head. After the monologue showed up, my imagination is more like @echelon describes - I can talk to/against myself, go off on variants of movies etc. I am not entirely sure, but I _think_ my visual imagination has gotten weaker though, and I find it harder to enjoy fantasy now. Could be just age though.
3. Can't seem to be able to turn it off, or control it entirely. The monologue is my worst critic - not a bad way, but it'd be nice for it to not _be_ there all the time.