In the Stormlight Archive series, most plants in the East freak out if you step near them: grass retreats into stone as you step on it, rockbuds pull their vines into their shells if you are close, etc. Are plants on Roshar ever shown to be sensitive enough to react to vibrations in the air, or do they only react to the ground shaking or themselves being directly touched?
Probably. I believe that there is at least one example of a plant responding to vibrations in the air, though it isn't airtight.
This answer is a little bit dicey because vibrations don't exist strictly in one medium versus another-- something vibrating the ground will almost certainly produce vibrations in the air as well, and vice-versa. But the example I have in mind is:
"Go down," Shallan whispered to Pattern, "and walk to the tree. Try to approach carefully, slowly, and buzz loudly to get the tree's leaves to pull back. (Words of Radiance, Ch. 63)
Shallan's instruction was specifically about sound-- she didn't say "shake the tree" or "touch the tree". Somewhat later, this plan (to make it appear as if someone startled the tree after approaching it) is shown to work as Iyatil leaves her hiding place to follow the person she thinks is Veil.
We know a couple of things about Pattern's manifestation in the Physical realm:
1. Unlike some other spren, his communication seems to be always audible to his surroundings.
Contrast with Syl, who can be selectively inaudible. Pattern can be subtle, but can't limit those who can hear him when he speaks (beyond adjusting his volume). This is relevant because if he speaks and is generally audible, it stands to reason that he's speaking in the normal way: by vibrating the air. This should be true whether he is speaking himself or Shallan is utilizing the audio aspect of the surge of illumination.
2. Pattern, like other spren, has very little capacity to affect physical objects in the Physical realm.
Pattern can move a tiny amount of mass, like tripping a small lock, but can't shift objects like spheres very much. This suggests that his ability to "shake" the tree, other than with his voice, is also quite limited. This is doubly relevant because we must consider that Pattern is sliding across the ground in this situation, and it's not clear if he can vibrate the air without also vibrating the surface on which he is located.
We have no in-text indications of Pattern's voice changing (outside of his choosing to do an impression), let alone changing based on his "substrate" (like Shallan's dress as opposed to stone, wood, or when floating in the air). But even if we assume that he is vibrating the ground as well as the air while buzzing near the tree, his ability to cause the physical substance of the ground to move is almost certainly very low.
We also know a (very) little bit about this specific tree:
The tree in question is "tame" and used to people, and is stated as being less responsive to stimuli which ordinarily cause plants to retract:
The tree was used to people, and didn't pull in its fronds as they passed it. If someone had approached carefully enough to avoid brushing the fronds, Shallan would have spotted them. If, instead, they'd moved quickly, the fronds would have felt the vibrations and withdrawn-which she also would have spotted. (Words of Radiance, Ch. 63)
It's not clear if this passage refers specifically to vibrations through the ground or vibrations in the air, or both. But it takes more than the typical incidental footstep to bother this tree-- a person making a careful approach would have to actually touch a frond to provoke a reaction. Per Shallan's instructions, Pattern isn't going to be touching the tree, and given (2) above he may not be able to produce vibration in the ground on the order of a non-cautious footstep.
More importantly, it is the fronds which are described as capable of feeling the vibrations, and any vibrations transmitted through the ground would be dampened by going through the other structures of the tree before affecting the fronds themselves.
So if Pattern was able to affect the tree (which we know he was), it seems more likely that he was able to do it by vibrating the air and making sound (which we know he can do with a reasonably loud volume) than by shifting the solid matter of the ground enough to make the fronds feel it and respond (we know he can't move physical objects much, perhaps at all).