Acquiring the skill of speaking in another language, or for that matter a child's learning to talk, does not follow a single recipe. People learn by variable amounts. A major component of speech learnability seems to be sensing precise feedback errors to correct subsequent utterances that help maintain speech goals. We have tested this idea in a speech motor learning paradigm under altered auditory feedback, in which subjects repeated a word while their auditory feedback was changed online. Subjects learned the task to variable degrees, with some simply failing to learn. We assessed feedback contribution by computing one-lag covariance between formant trajectories of the current feedback and the following utterance that was found to be a significant predictor of learning. Our findings rely on a novel use of information-rich formant trajectories in evaluating speech motor learning and argue for their relevance in auditory speech goals of vowel sounds.