I remember seeing a microphone shield somewhere, so I suggest you have a search for that and see what turns up. I expect it enabled the Arduino to detect sounds somehow or other. I have no idea how good the sound quality is, though, by the time it's reached the Arduino.
If you only want to make noises on the PC and aren't interested in connecting to any other VoIP services and aren't fussy about sound quality, you could get something crude working without a huge effort, I'd have thought. The sort of thing I have in mind is a mic connected to an Arduino being sampled at the best resolution/frequency combination you can achieve (I'd suggest 8-bit 8 KHz as a target), sending a byte stream over TCP/IP to the PC, where a custom application written by you would receive the byte stream and output the corresponding wave form to the PC's audio device. The PC side would be non-trivial, but it's all possible.