She does this even if she's just sitting and reading. There is no gurgle, but it is very noticeable. Sometimes it's more noticeable than other times. She takes meds every other day for vertigo. She is mentally sharp. Because she suffers from hearing loss, she does not hear herself breathing this way. Can anything be done?
Do you know what her wishes are for her medical care at this time? You say she is mentally sharp. Does she want to see her doctor?
It could be a sinus infection, cold, etc. You might discuss it with her doctor if you have Healthcare POA.
Check the side effects of the anti-vertigo meds; there might be some "critical mass" level as which use of them can cause nasal dryness or congestion.
My concern about mouth breathing would be that the amount of air intake isn't as great as it would be through nasal breathing. If you know how to breathe as a singer does, try that through the mouth and you'll find it's pretty limiting.