Skype is not available in the Ubuntu repositories but you can get hold of it (and a few other things) through medibuntu. However, it looks like that version of Skype has a problem with sound on Ubuntu 8.10 Intrepid Ibex, as detailed in this bug report. The workaround suggested in the discussion that consists in killing
pulseaudio before launching Skype works for me so that's good news. Even better news, it looks like the problem is fully sorted in Ubuntu 9.04, which is currently in beta. So, until then, I will use the workaround.
One lesson to learn from this though, is that if you ever have a problem with a piece of software on Linux, a good practice is to start that software from the command line: the output in the terminal window is invaluable for developers and maintainers to understand the problem and is a good place to start in order to find a solution to it. Quite often, just putting the error message in Google will return a number of articles and bug reports on that exact problem.