When striving to perform a prospective-memory (PM) task (i.e., remembering to perform an intended action), while concurrently performing an ongoing task, there are associated costs of PM (Smith, 2003). In laboratory experiments, these costs typically manifest as prolonged response times in the ongoing task. It has been proposed to investigate the underlying cognitive processes contributing to PM costs by modeling response times with an ex-Gaussian distribution (e.g., Ball & Brewer, 2018). It is assumed that the ex-Gaussian parameter τ reflects periodic monitoring for the appearance of a PM target (Rummel et al., 2017). To put the validity of this interpretation to test, we conducted an experiment (N = 176) with a standard laboratory event-based PM task and induced periodic monitoring by incorporating audio stimuli during the ongoing task that were similar to the PM cues. We included control groups who performed the ongoing task only, without PM instructions. We expected a positive effect of the audio manipulation on τ only when participants performed the PM task in addition to the ongoing task. Contrary to our hypothesis, auditory stimuli had no effect on parameter τ or the other ex-Gaussian parameters, although they did prolong response times. These findings suggest that periodic monitoring processes are not represented in the ex-Gaussian parameter τ.