You may be right Paddy but I think American English would use "sounds reasonable" the same way, implying that the tone doesn't make it appear that it came from a derranged person.  Usually, when somebody says "sounds reasonable," it's followed with quite a few qualifications, about how it isn't really reasonable, although it sounds that way.

But part of this discussion has to take into account that this medium isn't based on "sound," and you can actually read what is being said, something that is pretty valuable because it's easier to see the internal contradictions.

And if you do that, then  it isn't "reasonable," even in "tone"  and moves even further from dishonesty to deception, because it appears that the intent is to "sound reasonable" but actually be completely unreasonable, in a kind of new-agey, open, sort of dishonest way.  (We have a lot of that in California. )

And of course, Mike said he agreed with most of it, which is different than saying it "sounds" (appears) reasonable.