Hard of hearing people often only hear what they want to hear. Because of distortion in speech sounds, people with nerve hearing losses will often hear you 'talking but cannot understand what you are saying. To complicate matters, they often do not hear well enough to even realize that they missed what you said. Some letters are made inside the mouth or even in the throat, where they are totally invisible. It takes a great deal of concentration and energy.
Many people are poor speechreaders.
A hard of hearing person who has clear speech will have no trouble understanding you when you talk. Some hard of hearing people may have no noticeable affects on their speech. Others may have slight variations in their pronunciation that makes some people think they have accents. Still others may have speech that is difficult to understand. No matter how clearly a hard of hearing person speaks, never assume the hard of hearing person will understand you with equal ease.