Why do hearing-impaired people speak with a very nasal voice. Obviously our speech is, in part, determined by our ability to hear, but what specifically accounts for the nasality?
Nasal sounds are created when the velum, which is a sort of fleshy part of the roof of your mouth way back, is lowered to permit air to escape from the nose. Normally, the velum is raised during speech to produce oral sounds, but this is a habit that has to be learned; infants tend to nasalize sounds that shouldn’t actually be nasal, since when you’re not speaking the velum is lowered to permit breathing through the nose.
So I suspect the issue is that hard-of-hearing people might not have learned to properly make oral sounds, and without the feedback of being able to carefully compare their own voices to everyone else’s, they never learn to properly control the velum while speaking.