"Although it has not been proved, the suspicion is that plants can perceive sound through proteins that respond to pressure found within their cell membranes. Sound waves cause their leaves to vibrate ever so slightly, causing the plant to respond accordingly." Source: The Washington Post, Can plants hear? In a study, vibrations prompt some to boost their defenses, July 6, 2014
Plants need to be able to hear insects eating them to they can raise their defenses, so plants are already primed for the evolution of what we might call "ears." All that needs to happen is for these pressure-sensitive cells to cluster together. Then, it would make sense that a structure would form to better gather sound, which would be the exterior portion of the ear. In plants, this part would probably resemble a folded leaf, like a wolf's ear. Depending on the type of plant, the ear might be tightly rolled or loosely cupped. It might be smooth-rimmed like a lily leaf or jagged like an oak leaf.
If your plantimals can move away from danger, that would prompt the evolution of better ears than simple plants have. To a plant, the best it can do with the knowledge that something is coming to eat it is to make itself taste bad. While that's better than nothing, it's not exactly a sure-fire protection. If the plant could actually move away from the predator it would have much better chances of surviving, so a plant that could hear the predator coming before it was already there would have a major advantage.
I don't know exactly what "plant animals" means, but I would say that if you intend your plants to resemble animals, give them two swivel ears of leaves that resemble those of the base plant. If your plantimals are going to look like plants, set the ears close to the stem, probably in the crook of a branch.