We used a cross-modal priming paradigm to evoke a biphasic effect in visual short-term memory. Participants were required to match the memorandum (a visual shape, either spiky or curvy) to a delayed probe (a shape belonging to the same category). In two-thirds of trials the sequence of shapes was accompanied by a task-irrelevant sound (either tzk or upo, cross-modally correspondent to spiky and curvy shape categories, respectively). The biphasic effect occurred when a congruent vs. incongruent sound was presented 200ms after the memorandum, while it did not occur when the sound was presented 200ms before or simultaneously with it. The biphasic pattern of recognition sensitivities was revealed by an interaction between cross-modal congruency and probe delay, such that sensitivity was higher for visual shapes paired with a congruent rather than incongruent sound with a 300-ms delay, while the opposite was true with a 1300-ms delay. We suggest that this biphasic pattern of recognition sensitivities was dependent on the task-irrelevant sound activating different levels of shape processing as a function of the relative timing of sound, memorandum, and probe.