That's a bit of a strawman, don't you think?
> Ever used a modem?
Yes. And I don't think that's applicable in this case. First, a modem is designed specifically for such a use case--second, we're talking about an exploit that would (in theory) somehow make use of a microphone to spread itself to a target host.
You would have to assume that there's some flaw in either the audio driver for the microphone, firmware, or whatever that can be exploited by sending some combination of sounds to it. While that's possible, it seems unlikely that would be the case and much more likely that the culprit is an infected USB device of some sort, which has been discussed at length in other threads here (and would take far less work). For the variable quality of consumer microphones in a laptop or other such device to pick up a signal in such a manner as to exploit firmware/driver software at range makes this theory questionable IMO.
My personal hunch isn't that Mr. Ruiu is wrong. I think he's found something interesting and potentially dangerous, new, and fascinating. What I DO think, however, is that the Ars writer may have misunderstood, misinterpreted, or is misrepresenting the information he received.