Sound change is literally a change in a sound in a language. It does not necessarily happen in every dialect in the language, nor does it affect the word’s meaning or use. The term “sound change” applies to the historical evolution of the sounds of a language and is an umbrella term covering all types of sound changes. Each type of change has its own name, and the term “sound change” is too broad to give enough detail to identify what exactly happened.
Every language undergoes sound change, but not every type of sound change occurs in all languages. Different words can have multiple sound changes over time, changing later versions of a language so drastically that the old and modern versions are no longer mutually intelligible. In some cases, the two may seem like completely different languages.
Many of the changes involve a modification of one or more of the features of the sound. Each sound has different qualities that describe what goes into making the sound and what differentiates it from other sounds. The articulators, or parts of the mouth and throat used to produce the sound; the movement of the vocal folds; and the movement of air through the mouth or nose are three types of these features.
For example, the sound "f" is a voiceless labiodental fricative. This means the sound is produced with one lip and the upper teeth, the vocal folds are apart and making the sound voiceless, and air is able to pass out of the mouth with some friction. Changing one of those features transforms the "f" into another sound. For example, changing just the position of the vocal folds, from open to vibrating, changes the "f" to "v".
Sound change may involve either adding or subtracting sounds, combining sounds, strengthening or weakening them, switching them around or nasalizing them. It’s not unusual to see changes that mirror each other. Assimilation, for example, occurs when one sound changes to match an adjacent sound in some aspect, such as an "n" becoming voiced, or an "m", before a voiced consonant like "b".
The writing system of a language can suffer rather badly at the hands of a sound change because the spelling system doesn’t necessarily change along with the sound. Much of English spelling, particularly the vowels, doesn’t match up with the sounds because the vowel system has undergone huge shifts over time. One giant shift, known as the Great English Vowel Shift, occurred in the 15th to 17th centuries, changing vowels like the “ee” in “sheep.” That “ee” used to be a long “eh,” but the shift moved the articulatory position in the mouth up, so it now has the familiar “ee” sound.
Sounds do not reach one final point and stay that way. They might seem like they do that because a sound change is very gradual, but each sound can continue to change. As more dialects and languages come in contact with each other, as people start pronouncing sounds at slightly different places in the mouth and as children pick up on and adopt these modifications, society will see more sound changes occurring.