Audio conversation isn't implemented yet; aMSN is still growing and doesn't have all of MSN Messenger's features... yet. As I understand it, they're working on it and there's a possibility it'll be done in the next version.
Voice clips, meanwhile, are implemented in the latest SVN. See: http://www.amsn-project.net/wiki/SVN
Also, search the forum next time.. there's a topic on voice chat in the General forum on page 1.