A sexual act, which in managed via a microphone.
the participants talk dirty\in a suggestive language to each-other in order to arouse and excite their partner.
Any microphone can be used, usually plugged into a PC or a phone.
Or: well, I feel pretty horny and since we are on skype, and talkin, feel like having a bit of hot Micsex?
Tilly: Hell yea, get ready, here I come!