The pervasive use of information and computer technology in second or foreign language learning has led researchers to explore the ideal tasks for technological environments to facilitate second language (L2) learning. This study intended to contribute new knowledge to this area by examining the effects of the task complexity manipulated along the variable +-few elements in Robinson's Cognition Hypothesis on L2 production of 42 lower intermediate Chinese EFL (English as a Foreign Language) learners who completed two interactive tasks (simple versus complex) in dyads via audio chat of the video-conferencing platform WeMeet in a laboratory setting. Participants were also instructed to measure the difficulty of the tasks by responding to a self-rating questionnaire immediately after they completed each task. Their L2 output in the two tasks were recorded, transcribed and coded in three dimensions namely, syntactic complexity, lexical complexity and accuracy. SPSS 26 was used for statistical analyses. The results revealed that increasing task complexity induced significantly more lexically complex language. However, it did not result in significant changes in terms of syntactic complexity or accuracy of learners' L2 output via audio chat. These results contradicted the predictions of the Cognition Hypothesis, suggesting the inapplicability of Cognition Hypothesis in audio chat.