The phenomenon you're referring to is known as "rhotacism," which is the substitution or alteration of the sound represented by the letter 'r' in certain words. It's important to note that this change does not occur universally in all languages or dialects.
The variation in pronunciation of the 'r' sound in different words can be attributed to various factors, including phonetic patterns and historical linguistic developments. The specific changes you mentioned—'r' changing to 'l' in words like "restaurant" and "letter"—are examples of a linguistic process called "r-l metathesis." This metathesis occurs when sounds or letters within a word change places.
The reasons behind rhotacism and metathesis can be complex and vary depending on the specific word and language history. In the case of "restaurant" and "letter," the change from 'r' to 'l' likely occurred due to a combination of factors, including regional accents and the ease of pronunciation. In some dialects or accents, the pronunciation of 'r' and 'l' sounds can be similar, leading to occasional interchangeability.
On the other hand, in the word "armpit," the 'r' sound does not change to an 'n' sound. The pronunciation of "armpit" as a single word is actually the standard way of saying it, and there is no linguistic or historical reason for an 'n' sound to appear there. The 'r' sound and the 'p' sound are pronounced sequentially, without any significant alteration.
Language is a complex and evolving system, and the pronunciation of words can vary across regions and even between individuals. The changes in pronunciation you mentioned are specific to certain words and language patterns, and they do not represent a consistent transformation of 'r' into 'l' or 'n' in all contexts.