It wouldn't be simultaneous. The difference in great-circle lengths based on nonspherical oblateness (tens of miles) would account for up to a minute difference in arrival times at the antipode. At most it would be a sustained rumbling.
And on a global scale, there are many differences in pressure and wind speeds and temperature that do slightly alter the speed of sound. I don't know the magnitude of that, but it would further disperse the arrival times at the antipode to at least some extent.
They all cross the antipode, but not at the same time. There is no point where they all meet at the same time, if the planet is oblate rather than spherical, or has other irregularities in pressure or temperature or surface terrain.
And on a global scale, there are many differences in pressure and wind speeds and temperature that do slightly alter the speed of sound. I don't know the magnitude of that, but it would further disperse the arrival times at the antipode to at least some extent.