What noun would one use for a person who dominates as social conversation in a very loud voice? I have tried "monologist", but it doesn't sound appropriate. That sounds as though it refers to a profession.
That's a conversational narcissist:
A conversational narcissist will do their best to limit the discussion to topics where they are knowledgeable and can take up the most airtime.
In addition to both dominating the topic and airtime of a conversation, a narcissist might come across as incredibly arrogant and superior, making it even less likely that other people will cut in, for fear of being made to appear stupid, and thus enabling the narcissist to continue to monopolize everybody's time. (Men's Health)
It's a relatively new term (became common after 1970's)
Without further characterization of such a person you could refer to them as a "boisterous talker".
(SOED) boisterous 3 Of a person or action: rough or turbulent in behaviour or speech; (now esp.) exuberant, noisily cheerful
Complement after some more thinking about this question
Combining the answer of user Aled Cymro and the adjective shown above, a seeminly better term results.
- A boisterous monologist
(SOED) monologist A person who talks in a monologue or solioloquizes. Also, a person who monopolizes conversation.
Note I doubt there is a one-word term in which both ideas should be captured precisely enough.