In your audio samples, digital noise was evident at the transitions. Not on the final product, however. Your project had two blank clips at the end when viewed in Storybook mode. It's not obvious why.
Reworking the project, each file was placed on the timeline by itself (no others). Viewed in Storyboard mode, each had a .01 blank clip at its head.
When all three were dropped onto the timeline there was no blank between them, but instead a lengthy blank at the head. That was removed, using the f button to specify a zero duration fade.
With overlap mode the default, circle wipes were programmed at each transition. The wipes are seamless and no digital noise can be heard.
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