Hyponasality refers to a resonance disorder in which a person's speech sounds odd because too little air is escaping through the nose. This is the lesser known cousin of hypernasality.
Hyponasality is essentially the resonance/voice quality that you have when you have a stuffy nose.
The sounds /m/, /n/, and /ŋ/ (ng) are known as nasals because of the amount of sound energy released into the nasal cavity; as you might expect, these are greatly affected by hyponasality. The /m/ sound is likely to be replaced by /b/, the /n/ sound by /d/, and the /ŋ/ sound by /g/. However, in normal speech a certain amount of air passes into the nasal cavity where certain frequencies are amplified, and the absence of this resonance is noticeable in most speech sounds.
Temporary hyponasality is often caused by excessive mucus secretions and swollen mucus membranes, resulting from a cold, flu, or allergies. Chronic hyponasality may be caused by allergies or by structural deviations of the nasopharynx, nasal septum (such as a deviated septum), or sinus cavities. Other causes include growths such as nasal polyps, or enlarged adenoids.
Hyponasality is so commonly associated with swollen adenoids that there is a diagnostic label of 'adenoidal facies'. This consists of a cluster of signs that include swollen nose bridge, mouth breathing, pallor, dark circles under the eyes, nasal discharge, slow eating habits, and, of course, hyponasality.
Chronic hyponasality can cause some serious health problems even if the cause of the hyponasality is comparatively benign. Mouth breathing is a common result of hyponasality, which in extreme cases can cause developmental problems affecting dental occlusion, oral health, and perhaps even the proper growth of facial bones. Blockage of the Eustachian tubes commonly co-occurs with hyponasality, which can lead to chronic ear infections; chronic ear infections may cause permanent hearing loss and in severe cases may lead to meningitis. Blockage of the sinuses is also associated with a decreased sense of smell, which may lead to a decreased appetite. Theoretically lacking a sense of smell is also a health hazard because you can no longer detect gas leaks, smoke, and the like, but personally I'd worry more about the meningitis.
Correction of hyponasality will often result in temporary hypernasality, until the speaker learns to adjust their speech to account for the decreased nasal blockage. Speech should return to normal within six weeks after surgery, otherwise it's time for a another consult.
In the field of phonetics and phonetic transcription hyponasality is referred to as denasality.