When you call someone who is online via Google Talk, it use the back speaker in loud mode. Is there a way to use GTalk for calling like a regular phone call?
This may no longer be relevant since Voice is being replaced by Hangout, and you can only make video calls with Hangout. You could look into using a Google Voice number with a VoIP client, like Groove IP, to make calls. This is a great tutorial if you need some help getting started.