(this article is part of jason_ac's Intro To Myth guide. Visit the guide for more info)
Myth has text chat, but does not have built-in voice chat, and the volunteer developers are unlikely to add it. However, some have used Skype for voice chat.
There are other voice chat options (AIM, Yahoo, Mumble, Ventrilo), however they all have various drawbacks...they may not run on as many platforms, or they limit how many can be on a call (unless you pay), or they require someone to be running a server, etc.
To use Skype in random multiplayer pickup games: If you're hosting a game, start a Skype "group call". When someone joins your game lobby, give them your Skype contact info. They can add you as a Skype contact, then you can merge them into the existing group call. Now you'll have open voice chat for every person that's in the game. If you've got a buddy that you develop a verbal "code" with, you could even communicate plans to them in open chat.