When working in a professional setting, especially in administration or technical fields, common words/phrases/acronyms get used quite frequently. I've noticed that the more they are used daily, the faster they are physically spoken and the more relaxed the syllables become in the word, often making it sound completely different than when first used/spoken.
Is there a term for this phenomena?
I think it's just familiarity. I don't know a specific word for this phenomenon in relation to enunciation. The reason is simply that initially, speakers (or whoever they're talking to) may not be familiar with the term, so they say it more clearly.
Much the same thing happens in writing. When a writer introduces a new term, the first usage may be "quotated" or italicised to call attention to it. Thereafter he'll just write it unquotated, assuming you know what he means. Or if it's a long expression it may be written in full the first time, after which he'll use an abbreviation or acronym.
The only "technical" term I can think of to describe this is redundancy (sense 5a) which can be reduced when particular vocalisations / expressions are "expected", so they don't need to be so clearly differentiated from alternatives that aren't expected in the current context.
For individual sounds, see the term lenition.
A slight problem with the concept is that it might be argued that certain forms of articulation classed as being "weaker" by this theory may actually require more precise control of the articulators (e.g. keeping the articulators at just the right distance to cause frication may require more precision than a simple stop).
The answer has several parts, and is speculative, because I can't find any direct research on the topic.
First is 'lexical novelty'. We can look at studies like Children's Imitations of Lexical Items to see the process that children use when encountering new words. When the new acronym or phrase is used (especially for acronyms), the speaker is naturally more cautious of articulation, which might also act as a hedge in communication to be certain that the audience understands what is being said.
Once the speaker is familiar with the lexical item, or is more certain that the audience will understand what he means, then several phonetic processes can come into play. The most relevant would be Elision, Lenition, Assimilation and Apocope.
