A key problem in speech therapy is the motivation of patients in repetitive vocalization tasks. One important task is the vocalization of vowels. We present a novel solution by incorporating formant speech analysis into retro games to enable intrinsic motivation in performing the vocalization tasks in a fun and accessible manner. The visuals in the retro games also provide a simple and instantaneous feedback mechanism to the patients' vocalization performance. We developed an accurate and efficient formant recognition system to continuously recognize vowel vocalizations in real time. We implemented the system into two games, Speech Invaders and Yak-man, published on the iOS App Store in order to perform an initial public trial. We present the development to inform like-minded researchers who wish to incorporate real-time speech recognition in serious games.