Answer:
Feedback.
Explanation:
Noticing that, when giving a speech, many listeners are nodding in agreement (in this case, they could very well be shaking their heads in dissaproval), is a signal that is interpreted as positive. Every signal, unconscious or otherwise, be it a direct response or not, could be interpreted as feedback: a reaction, a response, a devolution. In this case, it's positive, as many listeners are nodding in agreement. But in a class context, for example, feedback could be given at the end of the speech to list things to improve, to acknowledge things that went well, etc etc.