For all others who stumble upon this post looking for a solution to this problem and aren't very Linux/Tech savy just like me, this was my solution and I'll try to explain it as simple as possible.
I run Lubuntu 12.04, and I had the same problem. As the others mentioned above, Skype 4.3 no longer supports the ALSA sound driver (Too bad! I really hope they bring support back for ALSA! I personally like ALSA quite a bit better than PulseAudio.) and now solely relies upon PulseAudio as it's audio driver. As far as I'm aware Lubuntu (at least Lubuntu 12.04) doesn't come with PulseAudio, so you'll need to install that yourself. I used this code to do so:
sudo apt-get install pulseaudio
Copy that into the LXTerminal (located in the "start" menu - Accessories) and press Enter. Once the process finishes, PulseAudio should be installed and you can check to see if your Skype audio is working now.