I am interested in development paths of Russian and English sound systems.
At present the situation is as follows: according to WALS, the consonant inventory of modern Russian is classed as "moderately large", while that of Modern English is considered "average". Russian palatalization has a lot to do with it but not only. There is an even bigger difference in vowel inventories (5/6 Rus.: about 21 Engl.), though WALS does not consider vowel lengths and diphthongs as the study is based on vowel qualities.
I am trying to go back to the start (i.e. to PIE) and pinpoint when and how (or possibly why) these differences occurred. Following my research I have already established that Proto-Germanic (which later evolved into Old English) is thought to have had nearly twice as many vowels as Proto-Slavic (which eventually gave life to Old Russian) but that number still is nearly a half of what Modern English has (if one counts diphthongs and trip thongs).
At the moment I am trying to find out how many vowels and consonants PIE language is thought to have had. PIE is not my specialism and I feel a bit lost as there are different opinions of various linguistic schools as well as a considerable number of small nuances. What I am trying to get is a very general idea of PIE sound inventory, sort of PIE phonology in a nutshell. I appreciate that PIE is a re-constructed language and different linguist proposed different theories about how it worked but has anybody ever come out with an approximate number for its possible vowels and consonants?