Why is audio quality so good (as reported by the people I speak with) in Skype-type voice chats even though I'm not using a headset?

There is a simple (depending on your definition of simple) feedback circuit that eliminates echo from speaker output. It's likely that your laptop subtracts the audio it sends to the speakers from the mic using a method like this. This avoids feedback, plus a nice microphone. No voodoo here.