With a growing number of applications involving social human-robot interactions, there is an increasingly important role for socially responsive speech interfaces that can effectively engage the user. For example, learning companions provide both task-related feedback and motivational support for students with the goal of improving learning. As a learning companion's ability to be socially responsive increases, so do learning outcomes. This paper presents a socially responsive speech interface for an embodied, robotic learning companion. We explore two methods of social responsiveness. The first method introduces social responses into the dialogue, while the second method augments these responses with voice-adaptation based on acoustic-prosodic entrainment. We evaluate the effect of a social, voice-adaptive robotic learning companion on social variables such as social presence and rapport, and we compare this to a companion with only social dialogue and one with neither social dialogue nor voice-adaptions. We contrast the effects against those of individual factors, such as gender. We find (1) that social presence is significantly higher with a social voice-adaptive speech interface than with purely social dialogue, and (2) that females feel significantly more rapport and are significantly more persistent in interactions with a robotic learning companion than males.