Acousto-optic imaging (AOI) enables optical-contrast imaging deep inside scattering samples via localized ultrasound-modulation of scattered light. While AOI allows optical investigations at depths, its imaging resolution is inherently limited by the ultrasound wavelength, prohibiting microscopic investigations. Here, we propose a computational imaging approach that allows optical diffraction-limited imaging using a conventional AOI system. We achieve this by extracting diffraction-limited imaging information from speckle correlations in the conventionally detected ultrasound-modulated scattered-light fields. Specifically, we identify that since ``memory-effect'' speckle correlations allow estimation of the Fourier magnitude of the field inside the ultrasound focus, scanning the ultrasound focus enables robust diffraction-limited reconstruction of extended objects using ptychography (i.e., we exploit the ultrasound focus as the scanned spatial-gate probe required for ptychographic phase retrieval). Moreover, we exploit the short speckle decorrelation-time in dynamic media, which is usually considered a hurdle for wavefront-shaping- based approaches, for improved ptychographic reconstruction. We experimentally demonstrate noninvasive imaging of targets that extend well beyond the memory-effect range, with a 40-times resolution improvement over conventional AOI.