Re: turn sour.
To "turn sour" means a bit more than "become less enjoyable". Simply becoming less enjoyable could mean that it was still enjoyable, but not as much as it had been before. If something is said to "turn sour", it means that it is not at all enjoyable any more - even that it has become repugnant, or unpalatable.
In this context, "tone" refers to both the colour [BrE spelling] and the style - and the whole atmosphere it engenders. He uses the term "overbearing" to give the same impression.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....