How do Americans pronounce the word 'now'?

There is no [æU] in IPA; IPA only uses lowercase symbols. X-SAMPA [{U] is identical to IPA [æʊ]. To see the conversion from X-SAMPA to IPA, just check out this site: http://en.wikipedia.org/wiki/X-SAMPA .

<<'Now' is pronounced "nau" almost everywhere in the English speaking world except in the interior of Canada where Canadian rounding causes it to sound like 'noh'.>>

????? What is Canadian rounding? What do you mean that it sounds like 'noh'? Could you use X-SAMPA or IPA please? I've never heard of such a thing, and I certainly wouldn't transcribe my 'now' diphthong as 'oh'. I say it essentially the same as most Americans: [aU], with a fronted [a_+].
Maybe you're thinking of Canadian raising, which raises the onset of this diphthong; for me, it's [EU] before voiceless consonants.
I also sometimes produce it as [a@] in casual speech, but I wouldn't write any of these as 'oh'.
I recorded myself saying this sound in a few different words:
http://media.putfile.com/MOUTH-set

I'd say I have [{U] for the "mouth" diphthong.

For the "mouth" diphthong, I have [aU], or more accurately [a_+U]. It's not quite [{U], but nonetheless the onset is noticeably more fronted than the onset of my "high" diphthong, [aI].