Yeah use Skype, currently bukkit or minecraft does not support in game talking and never will i think. Skype is free so heck why not
Technically a Spoutcraft client addon could be made that basically implements a voice client using Java and you could have some sort of server backend. Having the standalone server would probably be best for the backend.
if you need a ventrilo server, i own a public ventrilo with enough spare slots for another gaming community to use! http://slap-gaming.com/news/new-ventrilo-server for the connection info.