How does pronunciations become one of the main aspects of language change over time? Please describe it elaborately?

Several factors come together to cause diachronic change,
meaning, the change of a language or group of languages over time: language contact,
locations, time-saving, and different language
mediums.

Language contact: Through time,
speakers of a certain language will and have come into contact from speakers of a
different language.  This contact causes a borrowing of words, phrases, and meanings. 
For instance, the word "kindergarten," in which is commonly referred to as sometimes the
first level of education a child receives is actually borrowed from the German
language.

Location: When speakers of a language
become seperated by space (regions/countries/continents), their dialect changes to
accodomate their region.  For instance, this explains why the English language has many
different dialects (accents and language usage) between England, Ireland, Scotland, the
United States, Canada, and various other locales.  It's reflected by outside contact in
their region, regional events, and economic
advancement.

To save time:  This
factor, in regards to pronunication change, is self-explanatory.  Speakers of a certain
language will economize and shave off letters, parts of speech, syllables, and several
other things to save time and effort.  THis is how we get contractions such as "don't"
meaning "do not" and "can't" meaning cannot."  The English language made up for this by
creating the apostrophe, which reprensents something taken out of a
word.

Language mediums:  Pronunciation also
changes because different forms of communication are created, used, and discovered. 
Some mediums include writing, verbal language, electronic communication, and many
others.