First, for additional references, there is the original press release. Also, a similar report from a different black hole is here. It seems the Chandra people like this sort of thing. It is also worth noting that as far as I can tell, there are only press releases and no published scientific articles on this phenomenon.
Now to address the questions.
Black holes are so massive that light, which is faster than sound, can't escape.
Well, light from inside the black hole cannot escape. But active black holes create violent neighborhoods around them. In general, there will be an accretion disk - a relatively flat disk of material slowly spiraling into the black hole. Friction due to differential rotation in this disk can make it glowing hot, especially close to the black hole. Furthermore, angular momentum and magnetic fields conspire to make jets shooting out of the "poles" of the black hole (pretty much all black holes in astronomy are expected to be rotating significantly). Again, the matter never actually got into the event horizon, since by definition we would not see it again.
So what is this sound? As best I can tell from the press releases, these particular black holes cycle through periods of low accretion and high accretion. There may be a lot of material falling in for a long time, powering jets that pump energy out beyond even the galaxy in which the black hole resides.1 Then there will be a few million years of almost nothing falling into the black hole, during which the jets are little more than trickles. This cycle repeats semi-regularly, causing periodic bursts of energy to be sent out.
Sound can't travel in space (space has too much, well, space).
Well, yes and no. It is true there is some material even in the space between galaxies. On the other hand, it is extremely diffuse. In fact, it has been said (I cannot remember the source) that the densest clouds of material in interstellar space are more diffuse than the best vacuums we can make in laboratories. So you might imagine there can be some propagation, but not in the traditional sense.
Really, when the Chandra people say there is "sound," what they mean is this. Thus pulses of energy sent out form shock waves2 that propagate through the intergalactic medium. If you look far enough, you will see these periodic dense regions in a pattern not unlike the ripples in a pond. Since "sound" in the normal sense consists of overdensities traveling through the air (albeit without shocks in most cases), and since these are periodic overdensities in the diffuse gas between galaxies, we may as well make a connection between them in terminology.
It's a b-flat?
Take this with a large grain of salt. I doubt the duty cycle of the black hole is so regular as to produce a monochromatic "pitch." This is more of a whimsical calculation. Nathaniel in a comment did the reverse calculation - going from "note" to frequency. To see how the scientists changed a frequency $f$ (basically the reciprocal of the time between active periods of the black hole) to a named note, see this Wikipedia entry. In short, the number of the pitch is
$$ p = 69 + 12 \log_2\left(\frac{f}{440~\mathrm{Hz}}\right). $$
They plugged in an $f$ and must have gotten a $p$ around $-614$ or $-626$ (the wording is ambiguous). The B-flat above middle C has $p = 70$, the one below middle C has $p = 58$, the next one down is $p = 46$, etc.
Addendum: Why is this interesting? Beyond the whimsy, there is scientific value to this. Believe it or not, most of the mass (or at least, most of the mass of normal, non-dark matter) of a cluster of galaxies is found outside the galaxies themselves, in this intergalactic (aka intracluster) medium. It makes up a significant portion of the universe despite its low density, due entirely to the large volume it occupies. More interestingly, this gas is very hot - millions of degrees, despite the "background temperature" of the universe arguably being a few degrees above absolute zero. It is so hot that it glows in X-rays, which is why Chandra - a space-based X-ray telescope - studies it. These sound waves might explain a significant source of heating and injection of turbulence into this matter, which can have broad implications for extragalactic cosmology and galaxy formation. This all falls under the key term "AGN feedback," which is quite a hot topic in astrophysics these days.
1 Our own Milky Way's supermassive black hole probably had an active phase at some point. Astronomers recently detected the so-called Fermi bubbles that indicate two jets were shooting out of the center of our galaxy millions of years ago.
2 "Shock waves" have a technical meaning in fluid dynamics. Basically, there is a discontinuity in the density - ahead of the shock, everything is normal and diffuse, but as it passes there is almost instantaneous compression and heating. Think of the blast wave from an explosion.
