I am sure they are not interchangeable. I tried for 15 minutes yesterday to write a logical answer to your question and failed, so deleted my response. I will try again now...
One of the uses of por is to convey a motivating factor for an action. So in your example, the person talks all the time just to hear the sound of his/her voice. Hearing the voice is the cause or reason for talking.
One of the uses of para is to convey a goal. In the second example, the imperfect is used to talk about age. Talking about age is the goal of using the imperfect, so para.
The distinctions between these two examples, are in my mind, very subtle and I would be hard pressed to explain them convincingly. And maybe I am all wrong about this. But this is the best I can do.
Hopefully Cristian will explain things more clearly, and/or correct my mistaken explanation.