Hi, I thought I could expand on my original answer given some of the further questions asked here.

**Evil Eye**: sadly, no, sound can't escape a black hole. Ultimately sound is carried by gas particles, with each tiny particle colliding with the next in a chain reaction looking a bit like a mexican wave. So to get the sound out, each particle in turn would have to move a little bit outwards to hit the next in a long chain reaching out of the black hole. But it turns out particles inside a black hole have to move *towards* its centre -- they *cannot* move outwards. So not even sound can be transmitted out of a black hole.

**Geezer** asked if you could use a normal landline with a very long extension cord. Sadly, the same problem applies here: the electrons which carry the signal through the wire can only move inwards, not outwards. So, once again, the signal cannot be carried out of the black hole.

As to density, **Chris**, it's a little hard to give a straight answer as to how dense a black hole is. The answer normally given is that it's 'infinitely' dense -- that's because, as stated above, once infalling particles have passed the 'event horizon' they have to keep falling in, so ultimately they all reach the exact centre. The very centre becomes infinitely densely packed.

However, there are two problems with that answer.

First, most physicists think that at the very centre of black holes, our current theories of gravity are inadequate to describe the real universe. Corrections arise from 'quantum gravity' effects, and we cannot yet be sure of exactly what those effects do. They could, however, set some kind of upper limit on the density of matter, which from rough calculations we'd estimate to be the Planck Density: around 5 x 10^{96} kg/m^{3}.

Second, even disregarding quantum effects, it really depends on what you mean by the density. Because of the way time is distorted around and inside a black hole, you can get very different answers depending on exactly how you pose the question. (A related point is addressed in the Q&A in April's Naked Astronomy podcast.)

So here's another possible approach to an answer. Just take the mass of the black hole and divide by its apparent volume (that's a dangerous concept in itself, but I'm taking the size of the event horizon and making a naive calculation of the volume inside it). For a black hole of the mass of our sun, the nominal density would then be around 10^{19} kg/m^{3}. For the kind of super-massive black hole thought to be at the centre of our galaxy, the density would be rather less, something like 10^{6} kg/m^{3}. Odd that more massive black holes should be less dense, but that's just the way it goes.

And finally, neutron stars. The density is about 10^{17} kg/m^{3}. No, that's not a typo, it's really denser than the super-massive black hole. In a neutron star, sound travels at a significant fraction of the speed of light, perhaps between 5% and 75% (from 1.5 x 10^{7} m/s to 2 x 10^{8}m/s) depending on details of the modelling and whereabouts within the star you are talking about.