As is commonly known, many deaf people use speech reading to communicate with others. Some deaf people use it as primary means of communication, whereas others only use it when writing notes or using sign language isn't an option. Speech reading, unlike lip-reading, refers to understanding speech based on not only the movement of the lips, but facial expression, body language, and social context. Many lip-readers who have residual hearing also make great use of this hearing to make sense of lip movements. Speech reading skills vary from person to person, and aren't necessarily based on hearing loss levels, though residual hearing can be helpful. Speech reading is something that comes more naturally to some people than others, and that can be improved with practice for some but not all. Certain factors make it easier or harder for someone to speech read.
Good light, especially on the face, make it easier to speech read. On the other hand, if someone is silhouetted against another light source behind them (like the sun or a strong lamp), then it's impossible to see their face and therefore speech read.
It's easier to lipread someone who is closer than someone who is far away, for obvious reasons (a better view). If someone is walking around or keeps turning around (which is common in classroom situations) then it makes it impossible for someone to speech read.
2) Facial hair and lips
People with facial hair make it very hard or impossible to lipread because the facial hair is obstructing the lips. Lipstick can help make lipreading easier, especially for people with low vision.
3) Accents or mumbling
People with accents or people who mumble are hard to lipread because the way they pronounce words (or fail to) looks different on the lips.
4) Exaggerated speech
People who exaggerate their lip movements or over-anunciate make it impossible to read their lips because the shapes they are forming aren't the ones naturally made with spoken language.
5) Natural gesturing
Most people gesture or use their hands at least a bit when speaking. This can help convey meaning for deaf people who are relying on the entire body language to understand what's being said. Be careful not to use exaggerated gesturing because this can be considered obnoxious or rude, and isn't helpful to comprehension. Pointing to things you're talking about is reasonable though, such as pointing to the gravy when you're asking "do you want your mashed potatoes with or without gravy?"
6) Topic changes
Speech reading or understanding speech with hearing loss requires heavy reliance on context. This is because it's much like trying to read something with letters missing, where the gaps have to be "filled in." How to fill in the gaps depends on the topic. If a sudden topic change occurs, it can become hard for a speech reader to follow. It's helpful to inform someone speech reading of sudden topic changes.
Shouting doesn't help. The reason shouting doesn't help is because hearing loss not only effects how loud things sound, but also the clarity. When people shout, their speech often becomes even less clear than usual. It's also much harder to speech read someone who is shouting than someone who is speaking naturally.
Understanding Speech with Hearing Loss
Trying to understand speech with hearing loss takes a lot of mental energy and can be tiring after a short while. To make an analogy, speech reading is like landing in a country where you sort of speak the language but have to put more energy into understanding it. If you get tired, then it becomes much harder to understand. If you spend all day talking, you get tired and need some time alone or with your native language. The speakers you talk to most generally become the easiest to understand.
Well-paced, clear speech is generally easier to understand than fast and half-articulated speech. A lot of this works the same way for someone trying to understand speech with hearing loss. While they may be able to understand you fine, it does take an added mental effort. Just like with a foreign language, it's generally easier for a hard of hearing person to understand voices that they're more used to. And of course context helps a great deal. In a foreign country, if someone is handing you a cup while they say "coffee?" its much easier to understand. Likewise context helps a great deal with understanding speech.
To understand how speech and hearing loss work, hearing loss is defined by both frequencies and decibels. Frequencies relates to pitch, how low or high a note is, and decibels relates to volume, or how soft or loud something is. People with partial hearing loss can usually hear better in certain frequency ranges than other ones. Most deaf people even have a tiny bit of residual hearing, but it's not usually very useful. There are some completely deaf people too, though, who can't hear a thing.
For some people, they have some hearing in the range of speech, and for others, speech is completely out of their range of hearing. Obviously this will affect their ability to understand speech, along with their speech reading skills. Because of the complex nature of speech in terms of the frequency combinations, it's pretty common for someone to be able to hear environmental sound and music but have trouble understanding speech. It's important to keep in mind that just because someone can enjoy music or reacts to sounds doesn't mean they can understand what you're saying. There is a big difference between hearing speech and understanding it. Someone may be able to hear you speaking but it sounds garbled, muffled, or canned, so they can't make sense of what you're saying. Keep this in mind by not assuming that just because they can tell you're trying to speak to them, that they can understand you.
Yelling doesn't help because it only increases volume, often at the cost of losing clarity. Avoid the temptation to shout in deaf people's ears. Don't assume that a deaf person doesn't know you're shouting either. They can usually tell visually that you're yelling at them, and it can seem pretty insensitive even if that's not your intention.