Concrete walls are both - good and bad - for sound isolation. Good because of the sheer weight (can't get much heavier than that), so compared to other walls more energy is needed for the sound to enter the wall (the rest is reflected, which is bad for in-room accoustics). The bad part is that once the sound (vibration) has entered the wall there is pretty much no dampening whatsoever.
The worst thing is a solid concrete wall spanning multiple floors. No matter how high up you are, if someone scratches the wall with their fingernail in the basement, you will hear it if you're standing close to the wall.
If you compare STC-ratings for concrete walls (http://www.cement.org/tech/faq_stc.asp
) and drywall constructions (http://www.stcratings.com/assemblies.html
) you can see that the concrete wall is good, but by far not as good as one would expect from such a heavy material.
And yes, I'm speaking from experience. Germans are living in constant fear of drywall - if it's not concrete, brick or steel or better yet a combination of all three, it's not good for building. Unfortunately a lot builders skip the wall-decoupling between units, because that would cost money. During my years at university I lived in such a building. Neighbours laughing? I could hear it. Talking? Yep. I didn't understand what they were saying, but the voices were loud enough. (not necessarily the direct neighbours btw.) The worst things are "noise makers" in direct contact with the wall, also known as light switches. The constant clicking was driving me nuts sometimes.
Doesn't mean that concrete is bad per se, just have nothing vibrating in contact with it or even better, shield it with an additional decoupled wall, like some have done in their builds. Then you get the best of both worlds with absolutely amazing results.
P.S.: Both links above were just a quick google result, I have no idea how accurate they are.