The problem with a medication that might relax you enough so you feel like talking is that it would also relax you to a point that you don't talk much, even though you wouldn't mind doing so.

Perhaps you could ask your doc about something that would relax you only very slightly. But keep in mind it might not help you. Some people are just naturally quiet, mental illness or not.

I too talk more with lack of sleep but it is more of a disjointed rambling than an actual conversation.

I find drinking coffee (a stimulant) causes me to have a more natural chat with another person. That is if I've had enough sleep.