When only one voice is allowed to be heard then democracy is dead. What you have is a one party state. With no other opinions allowed to intrude on their fantasy world you can be sure the ideology of the regime will become increasingly bizarre, their laws increasingly oppressive.
The regime will virtue posture and act as though all other opinions are evil and that therefore they are justified in taking any measures no matter how violent or unjust against their enemies who hold such opinions. In reality it is the tyrant regime and its ideology that is evil.
The regime will pretend that it seized power and banned free speech to defend unpopular groups from having their feelings hurt by contrary opinions. In reality the regime bans free speech to make it difficult to organise political opposition to the regime.
The regime can never win. With no democratic oversight to correct defects the regime simply becomes more and more corrupt until it collapses under the weight of its own monstrous crimes.