Thanks for the explanation. I've heard of mutations in Welsh.

English can have aspirate mutations too. There's a song I know from Kentucky that's been written down this way:

   The pale ghost of the one I used to love --
   Why hit makes my blood run cold.

I believe that if you asked 1000 English speakers to say that (using standard spelling), then several hundred of them would put that h sound between 'why' and 'it.' And would that not be an aspirate mutation? Right here in our own back yard.