Ygorcs said:Cyrus, please understand this once and for all: people do add new sounds and forsake former sounds regardless of contacts with people who speak other languages. That's the internal development of languages. There's no need to "learn" new sounds from foreign nations when the gradual but continuous development of phonetics in a language already leads slowly to the creation or disappearance of phonemes, initially, generally, as allophones, but gradually spreading to replace others. You don't even have to have a degree in linguistics to that, even amateurs can notice that that can happen observing the recent development of their own language in an era with much more long distance contact and language standardisation as our own. Sounds change step by step into others because they already show the potential for that, all that it has to happen is a slight change in the articulation and the trick is done after some generations.
In almost all languages there were certainly some internal sound changes too, otherwise modern languages should be almost the same as ancient ones, but this process of simplification (like rd>l in Middle Persian or sk>sh in Old English) differs from the process of sound shifts between substrate and superstrate languages, it can't be denied that ancient Indo-Europeans migrated from a land to another land and sound changes in their languages mostly relate to substrate languages, for example it can't be said that native Brazilian languages had no role in nasal sounds in the Brazilian Portuguese.