Hello and welcome to the Captivate forum,
I do hope you did post in the right forum, because I'm a bit puzzled about your 'ActionScript 3' mention? Are you pointing to a Flash project or a Captivate project? If it is indeed a CP-project, OK. I will personally always record voice overs after having done the rest of the project: capturing, adding objects, timing etc. If you are working with Captivate 5 it has nice audio and speech features to create and synchonise your voice overs.