Recently, there has been much interest in both semi-supervised and manifold learning algorithms, though their applicability has not been explored for all domains. This paper has two goals: (i) to demonstrate semi-supervised approaches based solely on clustering are insufficient for phoneme classification and (ii) to present a new manifold-based semi-supervised algorithm to remedy this shortcoming. The improved performance of our approach over cluster-based methods substantiates the practical relevance of a geometric perspective on speech sounds.