Most machinima makers end up relying on voice talent they have contacted through the internet. Sims99 even has a very nice feature where voice actors can post audio resumés on their site, which is pretty spiffy. One of the biggest challenges with working with talent over distance is communicating what you want as a director regarding their vocal performance, and even then many times the recording is a "guess" of what is desired, and retakes are not uncommon, each separated by a barrier of bandwidth and time.

Strange Company recently used a different approach that is very interesting indeed. Over a Skype conference call, the director hooked up with the actors "live", and those actors recorded high-quality versions of their performances locally while the director listened in and coached in real time over VoIP. It probably felt a little clunky at first, as VoIP's latency can take a bit of getting used to, but I imagine in the long haul it saved them all a tremendous amount of time in getting the exact performance that was desired. Read all about it here.

Skype's basic VoIP services are FREE; you can contact other Skype users worldwide by voice at no cost other than what you pay for internet access. It's been a very big part of the collaboration of multiple teams I'm working with, and of course was and remains a central component of Machinima Live. The value of voice interaction with your collaborators, as I was doing last night instead of recording the overcast :) , cannot be overstated. And being that the service is free, there's really no excuse not to give it a try. Twenty bucks can get you a functional headset; twenty more can get you a very nice one.