I've discussed this with some associates but we couldn't come up with a solution. You might try a Skype forum.
Is the voice problem new, or has it never worked? Have you tried others, such as Yahoo Messenger? I'm trying to determine if it's a OS X problem or something else. One possibility is your ISP may be blocking voice services. Some do so you have to buy their voice service. For example, Verizon blocks all voice services on their cellular data network.
This is a silly Skype Upgrade bug. On a PowerPC when you've got Skype v.2.x and you (automatically or not) look for updates. It will find you an update, and it will tell you it's the latest 2.8 version, but in fact it will download the latest Intel only version. Which of course won't work on a PowerPC.
What you do is to go to the Skype web site, click on Get Skype - Mac, then you stay on top or go all the way down on the Mac page were you find Download Skype 2.8 for Mac and click on it, and then you download v. 188.8.131.526 and install it.
Then you open up Skype and go to Preferences (Menu - Skype - Preferences) were you uncheck "Check for updates automatically".
At least I've got no problems and I regularly use Skype on an iBook, much much better interface than the 5.x version that I've got on Windows. I use an USB headset to get a good sound.