The lack of an inner monologue seems linked to a lower ability to recall words and predict their sound.
How does this play for you ? I identify as spectrum, what used to be Asperger's, and have to work really hard to get to visual phantasia, but I can. Also worked hard to remove aural phantasia? via meditation because of negative self talk. Do you see 'aphant' as a useful designation? Thoughts, Ideas?
I don't need the voice. When I think of things, I see the thing and text spelling out the word of the thing in my mind. My internal landscape is pretty much like looking at a page in the encyclopedia. Aphantasia sounds so weird. Not seeing or hearing things in your mind, but you still have thoughts and can recognize things like any other person who visualizes in their head.
I have a pretty consistent internal voice, but I have partial aphantasia as I can't "picture" things. I think/remember in sensations and words. My best friend for years was an art student, and she was profoundly confused by me not having images in my head the way she did.
I started with medidating, deep relaxation. Laying flat works best for me. Discovered I can listen to the silence between the words. Then I was able to stretch out that silence until the words were so distant that they were mere reverberation. If there is a movie as well, that can be faded out. Later I learned a more daytime-compatible technique that is a bit hard to explain and could individually differently (opening the crown chakra).
In case it really ets your energy, and if you have access to it, consider talking to a therapist. You might be suffering from sommething like PTSD, depresseion or burn-out (in connection with autism), and those are really disorders that can be treated.