I think I'm at, if not indeed way past, the outer limits of what I know about the science of grammar. I think that's probably true in terms of oral/aural communication; possibly something that varies significantly with local dialects. The Welsh mutations are, of course, "rules" and an incorrect mutuation can change a word and occasionally be quite embarassing ...

Welsh has 9 consonants which can succumb to 1, 2 or all 3 mutuations: soft, nasal and aspirate. But it still isn't as difficult a language as people make it out to be! (Although looking words up in a dictionary can be a challenge!)

sian