This phenomenon is known as the occlusion effect. The voice distortion is so annoying to some hearing impaired people that they leave their hearing aids unused even though a few simple adjustments may take care of this problem in most cases.
The problem occurs when the hearing aid ear mould completely fills the ear canal trapping the vibrations from your own voice in the ear. Instead of escaping the ear, the vibrations bounce off the blockage and stay in the ear making the voice appear louder. In the lower frequencies the voice may be amplified as much as 20 dB.
In most cases, creating a vent through the hearing aid to unblock the ear solves the problem. This allows the vibrations of your own voice to escape the ear. A larger vent promotes a more normal sounding voice.
Some hearing aids come with a pressure relief vent, equalizing the air pressure in the ear, but these vents are often too small to reduce the occlusion effect. Thus, it may be necessary to create a larger passage in the hearing aid.
The occlusion effect can be measured during the hearing aid fitting. If the sound bothers the hearing aid user the dispenser at the clinic will be able to relieve the problem in most cases.
More adjustments may be necessary for satisfactory hearing aid performance, but the corrective steps make getting used to the hearing aid easier.
So-called open hearing aidds with extremely small ear moulds have eliminated much of the occlusion problem. But this type of hearing aid is suitable only for treatment of mild or moderate hearing loss.
Source: The "Occlusion Effect", Hearing Loss, 2004 No. 1.