So I read Comma Gets a Cure

The sound sample is definitely not by a Wisconsinite, as the low vowels are quite different than those you would find anywhere in Wisconsin. (It is almost certainly not by someone from here in southeastern Wisconsin, as the prosody is very different from that in the dialect here, and some of the non-low vowels are also noticably different in pronunciation.) My random guess is someone from either somewhere between, say, western Minnesota and Washington or someone from somewhere in Canada; I cannot really pin it down further than that.

What exactly are the "low" vowels, how do they differ from Wisconsin's low vowels, and what is "prosody"? I thought that had something to do with poetry.

The matter is that first, the speaker is not only cot-caught merged, which only shows up along the very northern and western extremes of Wisconsin, but also has the merged vowel generally as [ɒ] rather than [ɑ] or [a], which simply does not show up in Wisconsin. Likewise, historical /æ/ is quite low in the speech sample, almost approaching [a], which is very much unlike speech anywhere in Wisconsin.

As for prosody, I was specifically refer to the way syllables are timed and their overall rhythm, along with the general pattern of intonation. In the speech sample, the pattern of syllable timing and intonation were far too even compared to that of just about anyone I have heard from Wisconsin, with dialects here generally having significant variation in syllable timing corresponding to both stress, and in dialects with allophonic vowel length, consonants following vowels along with much stronger intonation.

What is the syllable timing in General American English?

It is hard to really explain in objective terms (without actually breaking out the spectrum analyzer software...), but GA has much more even timing of syllables than at least the dialect here, which gives much more time to more stressed syllables and much less time to less stressed syllables. Combined with the relative lack of intonation, GA sounds quite monotone and flat, and sometimes almost robotic, to my own ears...