The N800 has built in stereo speakers, as well as a built in microphone. However, for effective use of VOIP software, you really want a headset. The Nokia supplied headphones have stereo earbuds, as well as a built in microphone on the cord. The 1/8th inch audio jack will accept and work fine with any standard three conductor stereo headphones, but has contacts for a forth conductor to provide for a microphone as well.

Here is a picture of a standard heaphone connector (on the left) and the one on Nokia's headphone w mic combo (on the right).