Middle Ear Effusion: This condition, where fluid accumulates in the middle ear, is a common cause of muffled hearing. It often follows an upper respiratory infection and can be associated with a feeling of fullness in the ear.

Earwax Impaction: The accumulation of earwax can block the ear canal, leading to a sensation of muffled hearing. This is a common issue that can often be resolved with earwax removal.
Eustachian Tube Dysfunction: The Eustachian tube connects the middle ear to the back of the nose and throat. Dysfunction of this tube can lead to problems with equalizing the air pressure in the ear, resulting in muffled hearing.
Outer Ear Infection (Otitis Externa): An infection of the outer ear canal can cause swelling, discharge, and a feeling of fullness or muffled hearing in the affected ear.

Acoustic Neuroma: A rare, benign tumor on the nerve connecting the inner ear to the brain. Although uncommon, it's crucial to diagnose due to its potential impact on hearing and balance.
Middle Ear Tumor: Tumors in the middle ear, such as glomus tumors, are rare but can cause hearing loss and other symptoms like pulsatile tinnitus.
Stroke or Transient Ischemic Attack (TIA): In rare cases, sudden hearing loss, which might be perceived as muffled sound, can be a symptom of a stroke or TIA, especially if accompanied by other neurological symptoms.

Otosclerosis: A condition in which there is abnormal bone growth in the middle ear, which can cause hearing loss. It often starts with a conductive hearing loss that may be perceived as muffled sound.
Meniere's Disease: A disorder of the inner ear that affects balance and hearing, leading to episodes of vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear.
Autoimmune Inner Ear Disease: A rare condition where the body's immune system attacks the inner ear, leading to sudden hearing loss, which might be described as muffled sound, among other symptoms.