Hello Christina,
Thanks for reaching out!
I tested your project file using older versions of Storyline 360 (September 28, 2021 Build to most recent) and I can confirm that the pause you're hearing has been present ever since. The pause represents the program loading a different audio file which doesn't happen instantaneously, and there isn't a way to remove it using any built-in setting.
I'd recommend making the audio recording longer if possible to reduce the number of times that the program has to load it, which would effectively reduce the amount of pauses that the learner hears.